matcher: defmatch: useful :env parameter.

* share/txr/stdlib/match.tl (compile-match): Pattern macro
expanders now have an environment parameter. We turn the list
of variables that have been bound so far into a fake
macro-time lexical environment, the parent of which is the
surrounding environment. The pattern macro can query this
using the lexical-var-p function to determine whether a given
variable already has a binding, either in the pattern, or
in the surrounding lexical environment.
(defmatch): Generate a two-argument lambda, and use the new
mac-env-param-bind to make the environment object available
to the user-defined expansion.

* tests/011/patmatch.tl: New test cases for this environment
mechanism, and also for defmatch itself.

* txr.1: Document role of :env under defmatch.

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