* Bugfix: PEEK-CHAR always popped the unread-stuff, leading to
spurious duplicate echos in some cases.
* Minor incompatible change: UNREAD-CHAR on an ECHO-STREAM now unreads
onto the echo-stream's input stream. This is unspecified in the
CLHS, but makes SBCL compatible with most implementations (AFAICT,
everybody but CMUCL).
* Minor incompatible change: echo-streams used to buffer arbitrarily
many characters in UNREAD-CHAR. Conforming programs can't have
relied on this, but non-conforming ones might have; users who need
the old CMUCL/SBCL behavior can do it easily and de-facto-portably
with Gray Streams.
* Possible bugfix that nobody cares about: ECHO-N-BIN (which
implements a path through READ-SEQUENCE) can never have worked after
an UNREAD-CHAR, because it tried to store characters into an octet