0.8.20.6:

0.8.20.6:
	Make FILE-STREAM and STRING-STREAM potential mixins in CLOS
	(... and also, hackily, into structures.)
	... adjust FD-STREAM, and the various ANSI string streams.
	... sb-simple-streams can now make file-simple-stream and
		string-simple-stream subclasses (and hence subtypep)
		the relevant mixin.

	Caveat downloader I: This merge includes David Lichteblau's
	sb-simple-streams test suite extension, from which seven tests
	fail.

	Caveat downloader II: debug.impure.lisp is failing for me on
	x86/Linux.  I'm pretty sure this is not my fault -- I blame the
	debugger restructuring -- but it could be anyway.

	Caveat user: Bad Things happen if you try to mix both string-stream
	and file-stream into the same subclass.  Don't Do It.