`compact` and `compactValues`

[anandabits (Matthew Johnson)]

In the review thread @xwu commented:

@xwu

Note that _Unwrappable is already present in the standard library for bridging. However, one principal use case for such a protocol relies on the fact that only Optional conforms to it. This is essentially the case here also: it's meant to be a workaround for not being able to write a parameterized…

I replied: wouldn't _Unwrappable: Unwrappable be a viable workaround for that issue while allowing other types to conform to Unwrappable?