x86-64 disentwingling of fixnums and words.

x86-64 disentwingling of fixnums and words.

  * This is mostly constant fixups and supplying scaling factors in
places.

  * Where possible, I have used constructs that will simply generate the
correct code no matter what the width of a fixnum is.  In other places,
I have used an explicit check for the historic case and provided and
alternate code sequence for when it no longer applies.

  * Thanks to Paul Khuong for helping with the finding and fixing of
many of these places.