Revised FFI syntax + typing

[Marcin 'Qrczak' Kowalczyk]

I know that this is a silly argument, but Word is used internally in
RULES for fromIntegral to generate good code for conversions between
sized integers.

Since Word{8,16,32} and sometimes Word64 is represented as data-wrapped
Word#, it's easy to implement many conversions by delegation to the
conversion with source or target type replaced with Word.

This was actually what triggered me to add missing instances for Word
(Num and Integral were needed).

-- 
 __("<  Marcin Kowalczyk * qrczak at knm.org.pl http://qrczak.ids.net.pl/
 \__/
  ^^                      SYGNATURA ZASTÊPCZA
QRCZAK