SCL update + new cffi-sys primitive

- New cffi-sys primitive: canonicalize-symbol-name-case. Use this
  instead of read-from-string. Also new tests for this.
- cffi-scl.lisp: correct the evaluation order of %MEM-REF and %MEM-SET.
  Have 'foreign-symbol-pointer return 'nil if the symbol is not found.
- uffi-compat.lisp: better support for SCL.
- defcfun.lisp (defcfun.undefined): package at time of eval may differ
  from the compile time package.

Patch courtesy of Douglas Crosher.