Support added for the generation of rtl for open coded fixnum

Support added for the generation of rtl for open coded fixnum
arithmetic and predicates.
NOTE: Someday, when we have some support for the persistance of
untyped objects (i.e. when we have some way of saving unboxed
fixnums across GC calls) we can do some fancier things such as
eliminating the boxing of a fixnum calculation which is just going
to be unboxed and used in another fixnum calculation. My idea
for doing this is to add an explicit FIXNUM->OBJECT rtl expression
and when OBJECT->FIXNUM is simplified and its operand is FIXNUM->OBJECT
then cancel them out and return the operand of the FIXNUM->OBJECT.
The result of an fixnum expression expression will then have to be
considered an RTL:FIXNUM-VALUED-EXPRESSION? and OBJECT-NON-INVALIDATE
will have to be updated.