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.
- Made the `Any..Ext` implementations work with the repr of
  a trait object directly instead.
- Removed `Any`-related traits from the prelude.