std: Second-pass stabilization of `mem`

This commit takes a second pass through the `std::mem` module for stabilization.
The only remaining non-stable items in this module were `forget`, `transmute`,
`copy_lifetime`, and `copy_lifetime_mut`.

The `forget` and `transmute` intrinsics themselves were marked `#[stable]` to
propgate into the `core::mem` module so they would be marked stable.

The `copy_lifetime` functions were left `unstable`, but `Sized?` annotations
were added to the parameters to allow more general use with DSTs.

The `size_of_val`, `min_align_of_val`, and `align_of_val` functions would like
to grow `Sized?` bounds, but this is a backwards compatible change that
currently ICEs the compiler, so this change was not made at this time.

Finally, the module itself was declared `#![stable]` in this pass.