Support `MaybeUninit<T: ?Sized>` via `KnownLayout`

[joshlf]

This is prototyped in #1822.

This could help unify many standard library APIs that are currently duplicated, and make them more powerful.

For example, consider this duplication:

• https://doc.rust-lang.org/std/boxed/struct.Box.html#method.new_uninit
• https://doc.rust-lang.org/std/boxed/struct.Box.html#method.new_uninit_slice

This could be used to support an unsized MaybeValid<T> - #1833