New functions: SB!KERNEL:%FIXNUM-TO-TAGGED-WORD and %TAGGED-WORD-TO-FIXNUM.

These convert positive fixnums to and from unsigned words while not
actually modifying anything. Thus, the internal representation of fixnums
can be accessed at non-assembly level.
This can be useful for performing optimizations with tagged integers
in the source transform.
As of now, only the x86(-64) VOPs have been tested. There aren't any
architecture-related operations, however, so theoretically the
other implementations shouldn't have any problems either.