Features to support Ti50

TODO: Link these to the appropriate issues

- `MaybeValid` at least for `T: Sized`
- #1833
- https://github.com/google/zerocopy/issues/251
- Convenience method for going from `&[u8]` to `Option<&T>` (ie, shorthand for `Ref::new(bytes).map(Ref::into_ref)`)
- For tpm-rs:
- Slice DSTs
- Field projection inside `Unalign`
- Over-align `Align` type
- #249
- Need a type that doesn't have padding bytes so that you can go from `&T -> &Align`
- Need a type that *does* have padding bytes so that you can construct an `Align` by value
- #1852
- #1853
- #1855

cc @kupiakos