Add support for types in `std::num`.

[mintlu8]

This includes Wrapping<T>, Saturating<T>, NonZero<T>, NonZero* etc. I think parsing to and from numbers in FromLua and IntoLua without retaining these types' special property should be valid for most use cases, since it's crossing language boundaries.

[cheesycod]

@mintlu8 is this not possible already with mlua's serde ext