Removed lots (500 lines) of tagged-fixnum code. If you want it, use

Removed lots (500 lines) of tagged-fixnum code.  If you want it, use
RCS.  Changed the comments to reflect the current code.

Made things more robust:

 . Changed FITS-IN-nn-BITS? predicates to test for fixnum arguments.
 . Changed all the register*constant and constant*register predicates
   to be true only if the constant is a *fixnum* in the correct range.
 . Punted the GUARANTEE-SIGNED-FIXNUM (as the predicates now guarantee it).

The net effect is that the compiler now compiles code like (fix:+ x
1.2) or (fix:* 'a n) to the obvious, albeit meaningless, instructions
rather than signalling a confusing error.  Note that the midend
typerew phase can generate error messages for any of these conditions.