matcher: allow variables to back-reference.

Multiple occurrences of variables unify using equal.

* share/txr/stdlib/match.tl (var-list): New struct type.
Used for tracking what variables have been defined.
(compile-struct-match, compile-vec-match, compile-atom-match,
compile-op-match, compile-cons-structure,
compile-require-match, compile-let-match, compile-loop-match,
compile-parallel-match, compile-not-match): Take var-match
argument and pass it down.
(compile-parallel-match): Take var-match argument and pass
copies of it down to the compile jobs of the branches, since
they do not unify.
(compile-var-match, comiple-let-match, compile-op-match):
Handle variables carefully: check for variable already
being defined and generate a backreference instead of
a new binding match.
(compile-match): Take optional var-list argument,
instantiating the object if it is missing, pass down to all
recursive compile unctions.

* txr.1: Documented.

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