Change NoConstructors to a newtype over Void

[JordanMartinez]

Fixes #247

[kl0tl]

I don’t think we should do this: I doubt being able to eliminate terms of type NoConstructors with absurd is useful enough when we can already use unsafeCrashWith "unreachable" or unsafeCoerce.

[hdgarrood]

I do think that setting things up so that you don’t need to use unsafe functions is generally worth it, but since this doesn’t appear to be a breaking change we can deal with it after 0.14.0.

[kl0tl]

I agree, but here I'd prefer to not commit to this encoding yet so we can try to make some progress on purescript/purescript#2922.

[JordanMartinez]

Ok. I'll close this then. Thanks for the feedback.