Export `size_32/size_64` features from `rkyv`

[bonsairobo]

These features determine how usize is serialized, and they are mutually exclusive. If a downstream crate tries to enable size_64, they will get a compile error because, by default, parry enables size_32.

[bonsairobo]

This has become lower priority for me because it's easy enough to just not use usize in archived types.