rustdoc: Fix issues with cross-crate inlined associated items
* Visibility was missing from impl items.
* Attributes and docs were missing from consts and types in impls.
* Const default values were missing from traits.
This unifies the code that handles associated items from impls and traits.
Rustoc item summaries that are headers were not displayed at all because
they started with whitespace.
This PR fixes this and now removes the whitespace and then displays the
block.
* Visibility was missing from impl items.
* Attributes and docs were missing from consts and types in impls.
* Const default values were missing from traits.
This unifies the code that handles associated items from impls and traits.
rustdoc: include external files in documentation (RFC 1990)
Part of https://github.com/rust-lang/rfcs/pull/1990 (needs work on the error reporting, which i'm deferring to after this initial PR)
cc #44732
Also fixes#42760, because the prep work for the error reporting made it easy to fix that at the same time.
show in docs whether the return type of a function impls Iterator/Read/Write
Closes#25928
This PR makes it so that when rustdoc documents a function, it checks the return type to see whether it implements a handful of specific traits. If so, it will print the impl and any associated types. Rather than doing this via a whitelist within rustdoc, i chose to do this by a new `#[doc]` attribute parameter, so things like `Future` could tap into this if desired.
### Known shortcomings
~~The printing of impls currently uses the `where` class over the whole thing to shrink the font size relative to the function definition itself. Naturally, when the impl has a where clause of its own, it gets shrunken even further:~~ (This is no longer a problem because the design changed and rendered this concern moot.)
The lookup currently just looks at the top-level type, not looking inside things like Result or Option, which renders the spotlights on Read/Write a little less useful:
<details><summary>`File::{open, create}` don't have spotlight info (pic of old design)</summary>
</details>
All three of the initially spotlighted traits are generically implemented on `&mut` references. Rustdoc currently treats a `&mut T` reference-to-a-generic as an impl on the reference primitive itself. `&mut Self` counts as a generic in the eyes of rustdoc. All this combines to create this lovely scene on `Iterator::by_ref`:
<details><summary>`Iterator::by_ref` spotlights Iterator, Read, and Write (pic of old design)</summary>
</details>
