Add some Gray stream extensions
- Add some missing default Gray methods, like STREAM-READ-SEQUENCE etc.
- Fixup some existing default methods also.
- Generic versions of TRUENAME and PATHNAME. This is an optional part of the original Gray stream proposal. Recently added to ABCL and CLASP.
- SETF STREAM-ELEMENT-TYPE. This allows basic bivalent Gray streams. CCL already had this and I've added it to ABCL and Clasp recently.
- Add correct default methods for CLOSE and OPEN-STREAM-P. The default methods are supposed to store a "flag" indicating the open status to FUNDAMENTAL-STREAM.
I've tested both of these in my test suite.