Lispworks bugfix: %mem-ref and %mem-set compiler macros

- The %mem-ref and %mem-set in cffi-lispworks.lisp were using
  bogus indexes. FLI's documentation suggests foreign-typed-aref
  expects array indexes but it seems to want offsets in bytes
  instead.
- Regression tests: mem-ref.rt.1 and mem-ref.rt.2.