Improve safe transmute error reporting

This patch updates the error reporting when Safe Transmute is not
possible between 2 types by including the reason.

Also, fix some small bugs that occur when computing the `Answer` for
transmutability.

Author: bryangarza

core/src/mem/transmutability.rs

@@ -5,10 +5,6 @@
 /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
 #[unstable(feature = "transmutability", issue = "99571")]
 #[lang = "transmute_trait"]
-#[rustc_on_unimplemented(
-    message = "`{Src}` cannot be safely transmuted into `{Self}` in the defining scope of `{Context}`.",
-    label = "`{Src}` cannot be safely transmuted into `{Self}` in the defining scope of `{Context}`."
-)]
 pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
 where
     Src: ?Sized,