Use datatypeReturnKind for lifted/unliftedness info

[TeofilC]

Currently we figure out whether a type is lifted/unlifted by calling reifyType on it to get its Kind.
This works well, but the coming version of th-abstraction now comes with this logic built-in, so it would be good to transition to rely on that functionality rather than redefining it here.

A tricky edge case is runtime rep polymorphic newtypes, but we have a test case for that.