Fix handling of item names for HIR

- Handle variants, fields, macros in `Node::ident()`
- Handle the crate root in `opt_item_name`
- Rewrite `item_name` in terms of `opt_item_name`

I need this for both rust-lang#77820 and rust-lang#78082, so splitting it out into a separate PR so it can land early.

Fix handling of item names for HIR

- Handle variants, fields, macros in `Node::ident()`
- Handle the crate root in `opt_item_name`
- Rewrite `item_name` in terms of `opt_item_name`

I need this for both rust-lang#77820 and rust-lang#78082, so splitting it out into a separate PR so it can land early.

Fix handling of item names for HIR

- Handle variants, fields, macros in `Node::ident()`
- Handle the crate root in `opt_item_name`
- Rewrite `item_name` in terms of `opt_item_name`

I need this for both rust-lang#77820 and rust-lang#78082, so splitting it out into a separate PR so it can land early.

Make all rustdoc functions and structs crate-private

This gives warnings when code is no longer used, which will be helpful when rust-lang#77820 and rust-lang#78082 land.

AFAIK no one is using this API.

r? ``@GuillaumeGomez``
cc ``@rust-lang/rustdoc``

Get rid of clean::{Method, TyMethod}

They're redundant and almost the same as `clean::Function`.

I needed this for rust-lang#78082, although I forget why.

r? `@GuillaumeGomez`

Remove duplicate `Trait::auto` field

It was exactly the same as `is_auto`.

I found this while working on rust-lang#78082, but it's not required for that PR.

r? `@GuillaumeGomez`

Get rid of some doctree items

They can be derived directly from the `hir::Item`, there's no special logic.

- TypeDef
- OpaqueTy
- Constant
- Static
- TraitAlias
- Enum
- Union
- Struct

Part of rust-lang#78082 (the easiest part, I'm still debugging some other changes).
r? `@GuillaumeGomez`

Get rid of `doctree::Impl`

Follow-up to rust-lang#79264, continues breaking up rust-lang#78082. At some point I want to introduce `MaybeInlined`, but I think I'll wait until I need `MaybeInlined::InlinedWithOriginal` because it's not very useful in other situations.

r? `@GuillaumeGomez`

Get rid of doctree::{ExternalCrate, ForeignItem, Trait, Function}

Closes rust-lang#79314, closes rust-lang#79331, closes rust-lang#79332. Follow-up to rust-lang#79264 and rust-lang#79312, continues breaking up rust-lang#78082.

r? `@GuillaumeGomez`

Remove doctree::Macro and distinguish between `macro_rules!` and `pub macro`

This is a part of rust-lang#78082, removing doctree::Macro. Uses the changes in rust-lang#79372

Fixes rust-lang#76761

Pass around Symbols instead of Idents in doctree

The span was unused.

Vaguely related to rust-lang#78082 - currently working on converting `visit_ast` to use `hir::intravisit` and this makes that a little easier.

r? `@GuillaumeGomez`

Pass around Symbols instead of Idents in doctree

The span was unused.

Vaguely related to rust-lang#78082 - currently working on converting `visit_ast` to use `hir::intravisit` and this makes that a little easier.

r? ``@GuillaumeGomez``