Improve portability and fix compilation on CLISP

[bohonghuang]

On some implementations that support gray streams, such as CLISP, input-stream-p and output-stream-p are not generic functions, which makes the following code not compiled:

(defmethod input-stream-p ((stream fast-input-stream))
  (with-slots (buffer) stream
    (and (typep buffer 'input-buffer))))

(defmethod output-stream-p ((stream fast-output-stream))
  (with-slots (buffer) stream
    (and (typep buffer 'output-buffer))))

Indeed, input-stream-p and output-stream-p return t for any instance of fundamental-input-stream and fundamental-output-stream, so the above 2 methods are not required by other implementations where they are generic functions though.