Fixnum and unsigned-fixnum array cleanups.

Fixnum and unsigned-fixnum array cleanups.

  * Rename {arraytype}-{elementtype1} to {arraytype}-{elementtype2} for
{arraytype} in (SIMPLE-ARRAY VECTOR), {elementtype1} in
(UNSIGNED-BYTE-29 UNSIGNED-BYTE-60 SIGNED-BYTE-30 SIGNED-BYTE-61) and
{elementtype2} in (UNSIGNED-FIXNUM SIGNED-FIXNUM) across the board
(predicates, tag names, etc.).

  * Cleanup of related conditional compilation.

  * Export n-fixnum-bits (the remaining fixnum representation constant)
from sb!vm instead of having it as an internal symbol of sb!pcl.

  * Original patch by Paul Khuong.