Cleaned up `std::any`

- Added `TraitObject` representation to `std::raw`.
- Added doc to `std::raw`.
- Removed `Any::as_void_ptr()` and `Any::as_mut_void_ptr()`
  methods as they are uneccessary now after the removal of
  headers on owned boxes. This reduces the number of virtual calls needed.
- Made the `..Ext` implementations work directly with the repr of
  a trait object.
- Removed `Any`-related traits from the prelude.

- Added bench for `Any`
auto merge of #12667 : Kimundi/rust/any_improv, r=cmr

- Added `TraitObject` representation to `std::raw`.
- Added doc to `std::raw`.
- Removed `Any::as_void_ptr()` and `Any::as_mut_void_ptr()`
  methods as they are uneccessary now after the removal of
  headers on owned boxes. This reduces the number of virtual calls needed from 2 to 1.
- Made the `..Ext` implementations work directly with the repr of
  a trait object.
- Removed `Any`-related traits from the prelude.
- Added bench.

Bench before/after:
~~~
7 ns/iter (+/- 0)
4 ns/iter (+/- 0)
~~~
rustdoc: Add anchors to section headers

This commit adds a appear-on-over link to all section headers to generated
documentation. Each header also receives an id now, even those generated through
markdown. The purpose of this is to provide easy to link to sections.

This modifies the default header markdown generation because the default id
added looks like "toc_NN" which is difficult to reconcile among all sections (by
default each section gets a "toc_0" id), and it's also not very descriptive of
where you're going.

This chooses to adopt the github-style anchors by taking the contents of the
title and hyphen-separating them (after lower casing).

Closes #12681