matcher: bind some temporaries with let.

* share/txr/stdlib/match.tl (match-vars): New slots, pure-vars
and pure-var-exprs.
(match-vars lets): New method.
(compiled-match wrap-guards): Generate an alet that binds the
temporaries, and then does the assignments of the regular
variables.
(compile-vec-match, compile-cons-structure,
compile-hash-match, compile-range-match): Use the pure-vars
for match-guard temporaries that are bound and not assigned,
rather than vars.
(when-match, if-match): Use alet for obj-var.

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