This PR adds niche optimization. This lets the compiler use values
outside of the valid integer range to store e.g. other enum variants.
This makes `Option<u7>` one byte.

The internal representation is own the same as the related primitive
type, i.e. every u30 can be transmuted to u32. The other direction is
possible if the u32 is representable in the u30 range.

This speeds up the compilation by 50% (YMMV), because code is only
generated if used.

The methods should generate trivial instructions for most methods, so
there should be no drawbacks in using this annotation.