Add MaybeUninit type

The standard library's `MaybeUninit` type does not currently support
wrapping unsized types. This commit introduces a polyfill with the same
behavior as `MaybeUninit` which does support wrapping unsized types.

In this commit, the only supported types are sized types and slice
types. Later (as part of #29), we will add the ability to derive the
`AsMaybeUninit` trait, which will extend support to custom DSTs.

TODO: Figure out how to get rid of KnownLayout<MaybeUninit =
mem::MaybeUninit<T>> bounds.

Makes progress on #29