std: Stabilize unit, bool, ty, tuple, arc, any

[alexcrichton]

This commit applies stability attributes to the contents of these modules,
summarized here:

• The unit and bool modules have become #[unstable] as they are purely meant
  for documentation purposes and are candidates for removal.

• The ty module has been deprecated, and the inner Unsafe type has been
  renamed to UnsafeCell and moved to the cell module. The marker1 field
  has been removed as the compiler now always infers UnsafeCell to be
  invariant. The new method i stable, but the value field, get and
  unwrap methods are all unstable.

• The tuple module has its name as stable, the naming of the TupleN traits
  as stable while the methods are all #[unstable]. The other impls in the module
  have appropriate stability for the corresponding trait.

• The arc module has received the exact same treatment as the rc module
  previously did.

• The any module has its name as stable. The Any trait is also stable, with
  a new private supertrait which now contains the get_type_id method. This is
  to make the method a private implementation detail rather than a public-facing
  detail.

  The two extension traits in the module are marked #[unstable] as they will not
  be necessary with DST. The is method is #[stable], the as_{mut,ref} methods
  have been renamed to downcast_{mut,ref} and are #[unstable].

  The extension trait BoxAny has been clarified as to why it is unstable as it
  will not be necessary with DST.

This is a breaking change because the marker1 field was removed from the
UnsafeCell type. To deal with this change, you can simply delete the field and
only specify the value of the data field in static initializers.

[breaking-change]

[alexcrichton]

Drawing special attention to this file, it would be quite sad if I made an error!