Generate binding for function _addresses_

[edsko]

Suppose we have

void showInt(int);
void callFunPtr(void (*)(int));

and we want to be able to call callFunPtr, passing showInt as an argument. Then we will to generate

foreign import capi "example.h callFunPtr"
  cCallFunPtr :: FunPtr (Int -> IO ()) -> IO ()

foreign import capi "example.h &showInt"
  addrOf_cShowInt :: FunPtr (Int -> IO ())

Not sure if we simply want to generate bindings for all function addresses, subject to the regular selection rules, or whether we need a separate mechanism here.