Cleanup markdown span handling

1. Get rid of `locate()` in markdown handling

This function was unfortunate for several reasons:

- It used `unsafe` because it wanted to tell whether a string came from
  the same *allocation* as another, not just whether it was a textual match.
- It recalculated spans even though they were already available from pulldown
- It sometimes *failed* to calculate the span, which meant it was always possible for the span to be `None`, even though in practice that should never happen.

This has several cleanups:

- Make the span required
- Pass through the span from pulldown in the `HeadingLinks` and `Footnotes` iterators
- Only add iterator bounds on the `impl Iterator`, not on `new` and the struct itself.

2. Remove unnecessary scope in `markdown_links`

I recommend reading a single commit at a time.

cc `@bugadani` - this will conflict with rust-lang#77859, I'll try to make sure that gets merged first.

Rollup of 11 pull requests

Successful merges:

 - rust-lang#80159 (Add array search aliases)
 - rust-lang#80166 (Edit rustc_middle docs)
 - rust-lang#80170 (Fix ICE when lookup method in trait for type that have bound vars)
 - rust-lang#80171 (Edit rustc_middle::ty::TyKind docs)
 - rust-lang#80199 (also const-check FakeRead)
 - rust-lang#80211 (Handle desugaring in impl trait bound suggestion)
 - rust-lang#80236 (Use pointer type in AtomicPtr::swap implementation)
 - rust-lang#80239 (Update Clippy)
 - rust-lang#80240 (make sure installer only creates directories in DESTDIR)
 - rust-lang#80244 (Cleanup markdown span handling)
 - rust-lang#80250 (Minor cleanups in LateResolver)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

…llaumeGomez

Revert "Cleanup markdown span handling"

Reverts rust-lang#80244. This caused a diagnostic regression, originally it was:

```
warning: unresolved link to `std::process::Comman`
 --> link.rs:3:10
  |
3 | //! [a]: std::process::Comman
  |          ^^^^^^^^^^^^^^^^^^^^ no item named `Comman` in module `process`
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
```
but after that PR rustdoc now displays
```
warning: unresolved link to `std::process::Comman`
 --> link.rs:1:14
  |
1 | //! Links to [a] [link][a]
  |              ^^^ no item named `Comman` in module `process`
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
```
which IMO is much less clear.

cc `@bugadani,` thanks for catching this in rust-lang#77859.
r? `@GuillaumeGomez`

…llaumeGomez

Revert "Cleanup markdown span handling"

Reverts rust-lang#80244. This caused a diagnostic regression, originally it was:

```
warning: unresolved link to `std::process::Comman`
 --> link.rs:3:10
  |
3 | //! [a]: std::process::Comman
  |          ^^^^^^^^^^^^^^^^^^^^ no item named `Comman` in module `process`
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
```
but after that PR rustdoc now displays
```
warning: unresolved link to `std::process::Comman`
 --> link.rs:1:14
  |
1 | //! Links to [a] [link][a]
  |              ^^^ no item named `Comman` in module `process`
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
```
which IMO is much less clear.

cc `@bugadani,` thanks for catching this in rust-lang#77859.
r? `@GuillaumeGomez`

Cleanup markdown span handling, take 2

This PR includes the cleanups made in rust-lang#80244 except for the removal of `locate()`.

While the biggest conceptual part in rust-lang#80244 was the removal of `locate()`, it introduced a diagnostic regression.

Additional cleanup:
 - Use `RefCell` to avoid building two separate vectors for the links

Work to do:
- [ ] Decide if `locate()` can be simplified by assuming `s` is always in `md`
- [ ] Should probably add some tests that still provide the undesired diagnostics causing rust-lang#80381

cc `@jyn514` This is the best I can do without patching Pulldown to provide multiple ranges for reference-style links. Also, since `locate` is probably more efficient than `rfind` (at least it's constant time), I decided to not check the link type and just cover every &str as it was before.