* Completion of foreign import and foreign export for x86 ccall
  convention.  f-i's and f-x's can pass and return
  Char Int Word Addr StablePtr Float and Double.

* Significant cleanups and infrastructure improvements.
  Characterise functions by (instruction set, calling convention)
  pair where necessary, since that's what counts.

  Moved foreign export code into rts/ForeignCall.c.
  Should now be in a good position to implement x86 stdcall
  convention.