1.0.27.18: Changes to ECHO-STREAMs

1.0.27.18: Changes to ECHO-STREAMs

* 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
  buffer.