Make {integer}::from_str_radix constant

This commit makes FromStr on integers constant so that `const x: u32 = "23".parse();` works. More practical use-case is with environment variables at build time as discussed in rust-lang/rfcs#1907.

Tracking issue rust-lang#59133.

ACP: rust-lang/libs-team#74

…tr, r=dtolnay

Stabilize const `{integer}::from_str_radix` i.e. `const_int_from_str`

This PR stabilizes the feature `const_int_from_str`.

- ACP Issue: rust-lang/libs-team#74
- Implementation PR: rust-lang#99322
- Part of Tracking Issue: rust-lang#59133

API Change Diff:

```diff
impl {integer} {
- pub       fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
  pub const fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
}

impl ParseIntError {
- pub       fn kind(&self) -> &IntErrorKind;
  pub const fn kind(&self) -> &IntErrorKind;
}
```
This makes it easier to parse integers at compile-time, e.g.
the example from the Tracking Issue:

```rust
env!("SOMETHING").parse::<usize>().unwrap()
```

could now be achived  with

```rust
match usize::from_str_radix(env!("SOMETHING"), 10) {
  Ok(val) => val,
  Err(err) => panic!("Invalid value for SOMETHING environment variable."),
}
```

rather than having to depend on a library that implements or manually implement the parsing at compile-time.

---

Checklist based on [Libs Stabilization Guide - When there's const involved](https://std-dev-guide.rust-lang.org/development/stabilization.html#when-theres-const-involved)

I am treating this as a [partial stabilization](https://std-dev-guide.rust-lang.org/development/stabilization.html#partial-stabilizations) as it shares a tracking issue (and is rather small), so directly opening the partial stabilization PR for the subset (feature `const_int_from_str`) being stabilized.

- [x] ping Constant Evaluation WG
- [x] no unsafe involved
- [x] no `#[allow_internal_unstable]`
- [ ] usage of `intrinsic::const_eval_select` rust-lang#124625 in `from_str_radix_assert` to change the error message between compile-time and run-time
- [ ] [rust-labg/libs-api FCP](rust-lang#124941 (comment))

…tr, r=dtolnay

Stabilize const `{integer}::from_str_radix` i.e. `const_int_from_str`

This PR stabilizes the feature `const_int_from_str`.

- ACP Issue: rust-lang/libs-team#74
- Implementation PR: rust-lang#99322
- Part of Tracking Issue: rust-lang#59133

API Change Diff:

```diff
impl {integer} {
- pub       fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
  pub const fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
}

impl ParseIntError {
- pub       fn kind(&self) -> &IntErrorKind;
  pub const fn kind(&self) -> &IntErrorKind;
}
```
This makes it easier to parse integers at compile-time, e.g.
the example from the Tracking Issue:

```rust
env!("SOMETHING").parse::<usize>().unwrap()
```

could now be achived  with

```rust
match usize::from_str_radix(env!("SOMETHING"), 10) {
  Ok(val) => val,
  Err(err) => panic!("Invalid value for SOMETHING environment variable."),
}
```

rather than having to depend on a library that implements or manually implement the parsing at compile-time.

---

Checklist based on [Libs Stabilization Guide - When there's const involved](https://std-dev-guide.rust-lang.org/development/stabilization.html#when-theres-const-involved)

I am treating this as a [partial stabilization](https://std-dev-guide.rust-lang.org/development/stabilization.html#partial-stabilizations) as it shares a tracking issue (and is rather small), so directly opening the partial stabilization PR for the subset (feature `const_int_from_str`) being stabilized.

- [x] ping Constant Evaluation WG
- [x] no unsafe involved
- [x] no `#[allow_internal_unstable]`
- [ ] usage of `intrinsic::const_eval_select` rust-lang#124625 in `from_str_radix_assert` to change the error message between compile-time and run-time
- [ ] [rust-labg/libs-api FCP](rust-lang#124941 (comment))

…tr, r=dtolnay

Stabilize const `{integer}::from_str_radix` i.e. `const_int_from_str`

This PR stabilizes the feature `const_int_from_str`.

- ACP Issue: rust-lang/libs-team#74
- Implementation PR: rust-lang#99322
- Part of Tracking Issue: rust-lang#59133

API Change Diff:

```diff
impl {integer} {
- pub       fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
  pub const fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
}

impl ParseIntError {
- pub       fn kind(&self) -> &IntErrorKind;
  pub const fn kind(&self) -> &IntErrorKind;
}
```
This makes it easier to parse integers at compile-time, e.g.
the example from the Tracking Issue:

```rust
env!("SOMETHING").parse::<usize>().unwrap()
```

could now be achived  with

```rust
match usize::from_str_radix(env!("SOMETHING"), 10) {
  Ok(val) => val,
  Err(err) => panic!("Invalid value for SOMETHING environment variable."),
}
```

rather than having to depend on a library that implements or manually implement the parsing at compile-time.

---

Checklist based on [Libs Stabilization Guide - When there's const involved](https://std-dev-guide.rust-lang.org/development/stabilization.html#when-theres-const-involved)

I am treating this as a [partial stabilization](https://std-dev-guide.rust-lang.org/development/stabilization.html#partial-stabilizations) as it shares a tracking issue (and is rather small), so directly opening the partial stabilization PR for the subset (feature `const_int_from_str`) being stabilized.

- [x] ping Constant Evaluation WG
- [x] no unsafe involved
- [x] no `#[allow_internal_unstable]`
- [ ] usage of `intrinsic::const_eval_select` rust-lang#124625 in `from_str_radix_assert` to change the error message between compile-time and run-time
- [ ] [rust-labg/libs-api FCP](rust-lang#124941 (comment))

Rollup merge of rust-lang#124941 - Skgland:stabilize-const-int-from-str, r=dtolnay

Stabilize const `{integer}::from_str_radix` i.e. `const_int_from_str`

This PR stabilizes the feature `const_int_from_str`.

- ACP Issue: rust-lang/libs-team#74
- Implementation PR: rust-lang#99322
- Part of Tracking Issue: rust-lang#59133

API Change Diff:

```diff
impl {integer} {
- pub       fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
  pub const fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
}

impl ParseIntError {
- pub       fn kind(&self) -> &IntErrorKind;
  pub const fn kind(&self) -> &IntErrorKind;
}
```
This makes it easier to parse integers at compile-time, e.g.
the example from the Tracking Issue:

```rust
env!("SOMETHING").parse::<usize>().unwrap()
```

could now be achived  with

```rust
match usize::from_str_radix(env!("SOMETHING"), 10) {
  Ok(val) => val,
  Err(err) => panic!("Invalid value for SOMETHING environment variable."),
}
```

rather than having to depend on a library that implements or manually implement the parsing at compile-time.

---

Checklist based on [Libs Stabilization Guide - When there's const involved](https://std-dev-guide.rust-lang.org/development/stabilization.html#when-theres-const-involved)

I am treating this as a [partial stabilization](https://std-dev-guide.rust-lang.org/development/stabilization.html#partial-stabilizations) as it shares a tracking issue (and is rather small), so directly opening the partial stabilization PR for the subset (feature `const_int_from_str`) being stabilized.

- [x] ping Constant Evaluation WG
- [x] no unsafe involved
- [x] no `#[allow_internal_unstable]`
- [ ] usage of `intrinsic::const_eval_select` rust-lang#124625 in `from_str_radix_assert` to change the error message between compile-time and run-time
- [ ] [rust-labg/libs-api FCP](rust-lang#124941 (comment))