0.9.1.49: callbacks on x86

0.9.1.49: callbacks on x86
  * thanks to David Lichteblau for massaging the code originally
     ported to SBCL by Thomas F. Burdick, based on the work for CMUCL
     by Helmut Eller, to the current scheme of things.

  ...now what just the sparc backend remains to be ported from sbcl-callables,
  and a new one for mips...

  ...and getting the interface straight, and rebustifying the code a bit:
  sbcl-callables includes eg. some logic to check that the types given
  are compatible -- reinstating this sounds like a good idea one things
  settle down...