* Take advantage of new entry points in compiled code interface.

* Take advantage of new entry points in compiled code interface.
These changes require microcode 11.16 or later.

* Use BFEXTU instruction to extract type field.  This instruction is
both faster and smaller than the previous sequence, for both 6 and 8
bit types.  Use BFTST instruction to test for zero types.
Conditionalize use of the bit-field instructions on the new flag
`use-68020-instructions?'; I don't believe that we're using any other
68020-specific instructions besides these.

* Add rule for 1-arg fixnum predicates that tries to take advantage of
the preceding LSL.L instruction to test the number, rather than
emitting a redundant TST.L; this new rule is always used in generic
arithmetic expansions.