Introduce sb!vm::fixnum-lowtags

Introduce sb!vm::fixnum-lowtags

  * This is defined as a list of the exported SB!VM -LOWTAG symbols
bound to integers that are zero when masked with fixnum-tag-mask (in
short, the names of the fixnum lowtags).

  * Replace all direct references to the fixnum lowtags with something
based on fixnum-lowtags.

  * Introduce the corresponding change to genesis, with the predicate
is-fixnum-lowtag instead of testing against specific lowtags.

  * Introduce the corresponding change to the runtime, making fixnump()
check against fixnum-tag-mask instead of comparing individual fixnum
tags.

  * And, while we're redefining fixnump() in terms of the significant
part of the lowtag, do the same with other_immediate_lowtag_p().