General disentwingling of fixnums and words.

General disentwingling of fixnums and words.

  * Historically, n-fixnum-tag-bits has been equal to word-shift and has
been (1- n-lowtag-bits).  This led to implementors using constants and
calculations which happened to be right by coincidence rather than by
design.

  * Fix all places not part of the support for a particular backend to
use the defined-correct constants and calculations for the operations
being performed.

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