Add some more Gray extensions

[yitzchak]

1. Generic versions of TRUENAME and PATHNAME. This is an optional part of the original Gray stream proposal.
2. SETF STREAM-ELEMENT-TYPE. This allows basic bivalent Gray streams. CCL already had this and I've added it to Clasp recently.

I've tested both of these in my test suite. I'll push an update to CHANGES once CI finishes just in case I have broken something else.