As suggested by SimonM, moved an overview over the three components of the FFI
into the FFI document included as Chapter 7 into the GHC user guide.

Open questions:

* How can I make cross references to the doc in hslibs/?  There are already
  references to hslibs sections in other parts of ghc's user guide, but they
  are not correctly resolved when I generate html from the sgml files.

* There is now a (little) overlap with material in 6.5 (GHC Language Features:
  The foreign interface).  I would say, we could actually leave this as it is,
  as 6.5 goes on to describe GHC specifics.