SCL port, courtesy of Douglas Crosher

- Makefile: new test-scl target. Add SCL's fasl file extensions
  to the clean target.
- Remove SCL TODO item.
- New file: cffi-scl.lisp.
- New primitive type :long-double. Since it's only supported by
  SCL, it's not worth adding a no-long-double feature just yet.
- New tests for :long-double.
- Add information about SCL and the new :long-double type to
  the user manual.