Rewrite cons-specifier-ctype as generic function

[digikar99]

This is intended to fix #6

IIUC the issue about multiple return values for ctypep would be a different one. This extensibility at least allows for type specifiers with definite semantics - so ctypep returning a single return value works correctly for such specifiers, eg:

(defclass cstring= (ctype)
  ((string :initarg :string :reader cstring=-string :type string)))

(defun cstring= (string)
  (make-instance 'cstring= :string string))

(defmethod cons-specifier-ctype ((head (eql 'string=)) rest env)
  (declare (ignorable env))
  (destructuring-bind (string) rest
    (cstring= string)))

(defmethod unparse ((ct cstring=))
  `(string= ,(cstring=-string ct)))

(defmethod ctypep (object (ct cstring=))
  (and (stringp object)
       (string= object (cstring=-string ct))))

;;; so, (ctypep "qwerty" (specifier-ctype '(string= "qwerty"))) ;=> T

;;; and some more methods