Sort out promotion of unlifted types

Here's an unlifted type:

type T :: UnliftedType
data T = A | B Int

If I try to promote A:

type PA = 'A

I get an error: Couldn't match a lifted type with an unlifted type. I can fix this by adding a kind signature:

type PA :: T
type PA = 'A

If I try to promote B:

type PB = 'B

This works without a kind signature. If I add a kind signature with the kind that was inferred by GHC:

type PB :: Int -> T
type PB = 'B

then GHC rejects: Expected a lifted type, but ‘T’ is a boxed unlifted type.

I propose that for now we forbid:

1. Promotion of constructors of unlifted datatypes and newtypes
2. Promotion of unboxed tuples type T1 = '(# #), type T2 = '(#,#)
3. Unlifted type data declarations

type T :: UnliftedType
type data T = MkT

until there's motivation/theory behind it.

The other option is to at least be consistent and treat PA and PB in the same way.