Typechecking for "foreign import prim"

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.