Trait BikeshedGuaranteedNoDrop

Marker trait for the types that are allowed in union fields and unsafe binder types.

Implemented for:

• &T, &mut T for all T,
• ManuallyDrop<T> for all T,
• tuples and arrays whose elements implement BikeshedGuaranteedNoDrop,
• or otherwise, all types that are Copy.

Notably, this doesn't include all trivially-destructible types for semver reasons.

Bikeshed name for now. This trait does not do anything other than reflect the set of types that are allowed within unions for field validity.