Support for casting/bridging across separate template instantiations.

This CL reworks how the `forward_declare.rs` crate supports casting
across forward declarations and complete definitions:

1. This CL introduces a new `TransmutabilityEquivalenceClass` trait
   (which provides a single associated item - the `Name` type alias).
2. The `IncompleteTransmute<Target>` trait has been made private
   and reimplemented on top of the `TransmutabilityEquivalenceClass`
   trait:

   A. Structs/unions with the same
      `TransmutabilityEquivalenceClass::Name` can be safely transmuted
      between each other.
   B. Default `impl`s define separate equivalences classes for i)
      shared/immutable references `&T`, and ii) exclusive/immutable
      references `&mut T`, and iii) `Vec<&T>`.

The changes above are sufficient to support casting across separate
template instantiations (because Crubit already generates an appropriate
`!unsafe_define` macro call for template instantiations).  This CL adds
an explicit test under `test/templates/bridging` as well as a bit of
test coverage under `test/templates/type_alias`.

For readability/reviewability the CL tries to minimize the amount of
changes (i.e. avoiding removing, renaming, or reordering items in
`forward_declare.rs`).  A TODO has been left to follow-up with a
separate CL that will clean things up.

PiperOrigin-RevId: 458234228