std: Second pass stabilization for `boxed`

std: Second pass stabilization for `boxed`

This commit performs a second pass over the `std::boxed` module, taking the
following actions:

* `boxed` is now stable
* `Box` is now stable
* `BoxAny` is removed in favor of a direct `impl Box<Any>`
* `Box::downcast` remains unstable while the name of the `downcast` family of
  methods is determined.

This is a breaking change due to the removal of the `BoxAny` trait (note that
the `downcast` method still exists), and existing consumers of `BoxAny` simply
need to remove the import in their modules.

[breaking-change]