`NonMaxUsize` and niche value optimisation

[dlight]

@dlight

e #[repr(Rust)] and generally unavailable for inspection by FFI code). My rationale is that while NonMax is a special case that doesn't generalize, it's a very useful special case.

Then, maybe add NonN under the same rationale.

Whoa, that's even better!

Maybe a plan would be, stabilize some common and useful Non* types now on stdlib, and maybe expose the full generality of consecutive niches if and when pattern types become available.