[jhc] hmm. this approach to FFI export's types isn't working :-(.

[Samuel Bronson]

It seems that the types that would be appropriate for the arguments of
an FFI-exported function are not really allowed to exist in E or Grin
:-(. What should I do about this?

Perhaps the marshalling should be saved for the very end, and done in
C.FromGrin2 or thereabouts?