Stabilize `str::len`, `[T]::len` and `str::as_bytes` as const fn

r? @Centril

cc @RalfJung

This also introduces a scheme for making certain feature gates legal in stabilized const fns

Pkgsrc changes:
 * Remove patch which no longer applies (but what about RPATH?)
 * Adapt a few patches to changed files upstream.

Upstream changes:

Version 1.39.0 (2019-11-07)
===========================

Language
--------
- [You can now create `async` functions and blocks with `async fn`,
  `async move {}`, and `async {}` respectively, and you can now call
  `.await` on async expressions.][63209]
- [You can now use certain attributes on function, closure, and function
  pointer parameters.][64010] These attributes include `cfg`, `cfg_attr`,
  `allow`, `warn`, `deny`, `forbid` as well as inert helper attributes used
  by procedural macro attributes applied to items. e.g.
  ```rust
  fn len(
      #[cfg(windows)] slice: &[u16],
      #[cfg(not(windows))] slice: &[u8],
  ) -> usize {
      slice.len()
  }
  ```
- [You can now take shared references to bind-by-move patterns in the
  `if` guards of `match` arms.][63118] e.g.
  ```rust
  fn main() {
      let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]);

      match array {
          nums
  //      ---- `nums` is bound by move.
              if nums.iter().sum::<u8>() == 10
  //                 ^------ `.iter()` implicitly takes a reference to `nums`.
          => {
              drop(nums);
  //          ----------- Legal as `nums` was bound by move and so we have ownership.
          }
          _ => unreachable!(),
      }
  }
  ```

Compiler
--------
- [Added tier 3\* support for the `i686-unknown-uefi` target.][64334]
- [Added tier 3 support for the `sparc64-unknown-openbsd` target.][63595]
- [rustc will now trim code snippets in diagnostics to fit in your terminal.]
  [63402] **Note** Cargo currently doesn't use this feature. Refer to
  [cargo#7315][cargo/7315] to track this feature's progress.
- [You can now pass `--show-output` argument to test binaries to print the
  output of successful tests.][62600]

\* Refer to Rust's [platform support page][forge-platform-support] for more
information on Rust's tiered platform support.

Libraries
---------
- [`Vec::new` and `String::new` are now `const` functions.][64028]
- [`LinkedList::new` is now a `const` function.][63684]
- [`str::len`, `[T]::len` and `str::as_bytes` are now `const` functions.][63770]
- [The `abs`, `wrapping_abs`, and `overflowing_abs` numeric functions are
  now `const`.][63786]

Stabilized APIs
---------------
- [`Pin::into_inner`]
- [`Instant::checked_duration_since`]
- [`Instant::saturating_duration_since`]

Cargo
-----
- [You can now publish git dependencies if supplied with a `version`.]
  [cargo/7237]
- [The `--all` flag has been renamed to `--workspace`.][cargo/7241] Using
  `--all` is now deprecated.

Misc
----
- [You can now pass `-Clinker` to rustdoc to control the linker used
  for compiling doctests.][63834]

Compatibility Notes
-------------------
- [Code that was previously accepted by the old borrow checker, but rejected by
  the NLL borrow checker is now a hard error in Rust 2018.][63565] This was
  previously a warning, and will also become a hard error in the Rust 2015
  edition in the 1.40.0 release.
- [`rustdoc` now requires `rustc` to be installed and in the same directory to
  run tests.][63827] This should improve performance when running a large
  amount of doctests.
- [The `try!` macro will now issue a deprecation warning.][62672] It is
  recommended to use the `?` operator instead.
- [`asinh(-0.0)` now correctly returns `-0.0`.][63698] Previously this
  returned `0.0`.

[62600]: rust-lang/rust#62600
[62672]: rust-lang/rust#62672
[63118]: rust-lang/rust#63118
[63209]: rust-lang/rust#63209
[63402]: rust-lang/rust#63402
[63565]: rust-lang/rust#63565
[63595]: rust-lang/rust#63595
[63684]: rust-lang/rust#63684
[63698]: rust-lang/rust#63698
[63770]: rust-lang/rust#63770
[63786]: rust-lang/rust#63786
[63827]: rust-lang/rust#63827
[63834]: rust-lang/rust#63834
[63927]: rust-lang/rust#63927
[63933]: rust-lang/rust#63933
[63934]: rust-lang/rust#63934
[63938]: rust-lang/rust#63938
[63940]: rust-lang/rust#63940
[63941]: rust-lang/rust#63941
[63945]: rust-lang/rust#63945
[64010]: rust-lang/rust#64010
[64028]: rust-lang/rust#64028
[64334]: rust-lang/rust#64334
[cargo/7237]: rust-lang/cargo#7237
[cargo/7241]: rust-lang/cargo#7241
[cargo/7315]: rust-lang/cargo#7315
[`Pin::into_inner`]: https://doc.rust-lang.org/std/pin/struct.Pin.html#method.into_inner
[`Instant::checked_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_duration_since
[`Instant::saturating_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.saturating_duration_since

…r=Centril

Don't require `allow_internal_unstable` unless `staged_api` is enabled.

rust-lang#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely.

After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable.

r? @oli-obk (author of rust-lang#63770)
cc @Centril

…r=Centril

Don't require `allow_internal_unstable` unless `staged_api` is enabled.

rust-lang#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely.

After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable.

r? @oli-obk (author of rust-lang#63770)
cc @Centril

…r=Centril

Don't require `allow_internal_unstable` unless `staged_api` is enabled.

rust-lang#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely.

After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable.

r? @oli-obk (author of rust-lang#63770)
cc @Centril