Implement the C99 complex type

[Luís Oliveira]

Suggested by Tamas K. Papp.

Prototype code:

(defmethod translate-from-foreign (ptr (type complex-float-type))
 (complex (mem-aref ptr :float 0) (mem-aref ptr :float 1)))

(defun complex-float-aref (ptr index c)
 (let ((p (* (foreign-type-size :complex-float) index)))
   (setf (mem-aref p :float 0) (realpart c)
           (mem-aref p :float 1) (imagpart c))))