`Invalid` trait for space optimization of enums.

[rust-highfive]

Proposing to add Invalid trait and use it for Option<T> space optimization. This is an alternative to RFC #36.

Tracking issue for postponed PR #41

[kennytm]

Since we already have Zeroable trait, this issue could be closed?

[nrc]

Closing in favour of NonZero tracking issue

[aidanhs]

Actually closing :)