Operations on 8-bit and 12-bit operands shuffle args into range

Operations on 8-bit and 12-bit operands shuffle args into range

* module/language/cps/slot-allocation.scm (allocate-slots): Avoid
  allocating locals in the range [253,255].

* module/system/vm/assembler.scm: List exports explicitly.  For
  operations with limited-range operands, export wrapper assemblers that
  handle shuffling their operands into and out of their range.
  (define-assembler): Get rid of enclosing begin.
  (shuffling-assembler, define-shuffling-assembler): New helpers to
  define shuffling wrapper assemblers.
  (emit-mov*, emit-receive*): New functions.
  (shuffle-up-args): New helper.
  (standard-prelude, opt-prelude, kw-prelude): Call shuffle-up-args
  after finishing.

* test-suite/tests/compiler.test ("limits"): Add test cases.