Cleaning up the foreign object naming mess:

   * Renamed the primop from makeForeignObj# to mkForeignObj#, this
     is more consistent with the old Foreign.mkForeignObj.

   * PrelForeign now exports makeForeignObj with the new signature.

   * freeFile.c now uses StgAddr instead of StgForeignObj, this
     removes the need for some fixIOs.

   * Lots of import tweaking to prepare The Big Foreign Renamer,
     which will move most of Foreign to ForeignObj, and FFI to Foreign.

Note: I've tried to track the changes in the interpreter sources, but
didn't test them.