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.
std: Second pass stabilization of `default`

This commit performs a second pass stabilization of the `std::default` module.
The module was already marked `#[stable]`, and the inheritance of `#[stable]`
was removed since this attribute was applied. This commit adds the `#[stable]`
attribute to the trait definition and one method name, along with all
implementations found in the standard distribution.
Merge pull request #19902 from alexcrichton/second-pass-mem

std: Second-pass stabilization of `mem`/`default`

Reviewed-by: aturon