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.