It lints against features that are inteded to be internal to the
compiler and standard library. Implements MCP #596.

We allow `internal_features` in the standard library and compiler as those
use many features and this _is_ the standard library from the "internal to the compiler and
standard library" after all.

Marking some features as internal wasn't exactly the most scientific approach, I just marked some
mostly obvious features. While there is a categorization in the macro,
it's not very well upheld (should probably be fixed in another PR).

We always pass `-Ainternal_features` in the testsuite
About 400 UI tests and several other tests use internal features.
Instead of throwing the attribute on each one, just always allow them.
There's nothing wrong with testing internal features^^

Fixes #105906

This is common syntax used for intra-doc links, so fixing it should help
with doc formatting.

This code uses a special `hideme` class anyway, so just style that.

Fixes #100994

This selector was added in c7312fba.
The bug can be seen at <https://doc.rust-lang.org/1.27.0/alloc/slice/trait.SliceIndex.html#foreign-impls>.

This rule was added to help with a `<table>` that was used for displaying the
function signature [src] lockup. That lockup was changed in
34bd2b84 to use flexbox instead, leaving this
selector unused (at least, for its original purpose).

We recently removed the "up here" arrows on item-infos, and adjusted
vertical spacing so that even without the arrow, it would be visually
clear which item the item-info belonged to. The new CSS styles for
vertical spacing only applied to toggles, though. This missed
non-toggled impl blocks - for instance, those without any methods, like
https://doc.rust-lang.org/nightly/std/marker/trait.Send.html#implementors.
The result was lists of implementors that were spaced too closely. This
PR fixes the spacing by making it apply to non-toggled impl blocks as
well.

This also fixes an issue where item-infos were displayed too far below
their items. That was a result of display: table on .item-info .stab.
Changed that to display: inline-block.

This slightly reduces the noisiness of doc pages, making them easier to
read.

And add a rustdoc-gui test confirming various header sizes.

Detects https://github.com/rust-lang/rust/issues/88545