Rollup merge of #80244 - jyn514:spans, r=bugadani

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 https://github.com/rust-lang/rust/pull/77859, I'll try to make sure that gets merged first.

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