awk macro: support regexes better in ranges.

* share/txr/stdlib/awk.tl (sys:awk-compile-time): New slot,
rng-rec-temp. Specifies the name of a temporary variable
which is scoped over the evaluation of ranges, and which
caches a reference to the current record string.
(sys:range-test): New function.
(awk:let): Rename the original rng macrolet to sys:rng,
and introduce rng as a wrapper around it which inserts
the logic for calling the regex or function that might
emerge from the evaluation of from-expr or to-expr.
(awk): If ranges are present in the syntax, then bind
the temporary variable which caches the record around
the evaluations of the ranges.

* txr.1: Document new special behavior of rng w.r.t
functions and regexes. Add admonition against modifying
rec and some other awk variables from range expressions.

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