Typechecking for "foreign import prim"
The main restriction is that all args and results must be unboxed types. In particular we allow unboxed tuple results (which is a primary motivation for the whole feature). The normal rules apply about "void rep" result types like State#. We only allow "prim" calling convention for import, not export. The other forms of import, "dynamic", "wrapper" and data label are banned as a conseqence of checking that the imported name is a valid C string. We currently require prim imports to be marked unsafe, though this is essentially arbitrary as the safety information is unused.