document & impl the transmutation modeled by BikeshedIntrinsicFrom

Documents that `BikeshedIntrinsicFrom` models transmute-via-union,
which is slightly more expressive than the transmute-via-cast
implemented by `transmute_copy`. Additionally, we provide an
implementation of transmute-via-union as a method on the
`BikeshedIntrinsicFrom` trait with additional documentation on
the boundary between trait invariants and caller obligations.

Whether or not transmute-via-union is the right kind of transmute
to model remains up for discussion [1]. Regardless, it seems wise
to document the present behavior.

[1] https://rust-lang.zulipchat.com/#narrow/stream/216762-project-safe-transmute/topic/What.20'kind'.20of.20transmute.20to.20model.3F/near/426331967

Author: jswrenn