This is done by a 'unarisation' pre-pass at the STG level which
translates away all (live) binders binding something of unboxed
tuple type.

This has the following knock-on effects:
  * The subkind hierarchy is vastly simplified (no UbxTupleKind or ArgKind)
  * Various relaxed type checks in typechecker, 'foreign import prim' etc
  * All case binders may be live at the Core level