Implement compiler macros for %MEM-REF and %MEM-SET in LispWorks.

- Use FLI:FOREIGN-TYPED-AREF if available and dereferencing a number.
- Avoid calling INC-POINTER when the offset is a multiple of the type
  size allowing direct use of the INDEX argument to FOREIGN-TYPED-AREF.
- Fall back to open-coding the call to FLI:DEREFERENCE otherwise.