txr: real solution for spurious retention problem.

We take advantage of the nested, recursive nature of the
pattern language. Whenever a new data context is initiated, we
indicate that that context is current, in a dynamic variable.
Then, in the various data scanning directives (scan, collect,
repeat, gather), when performing the c->data = rest(c->data)
step to march down the lazy list, check whether the context c
is the current one. If that is the case, we know that
backtracking is not possible, and so we can safely pass the
c->data cons to the rcyc_cons function for recycling.
Otherwise we do the c->data = rest(c->data) only.
It's already the case that whenever backtracking is necessary,
a new, disposable context is allocated which is thrown away on
failure. Those contexts are never registered as current, and
so never match.

* match.c (match_files_ctx): Introduce a tag name to the
structure, struct match_files_ctx. Remove the volatile
qualifier from the data member, and put it back int the
original order. New member up for linking these structures
into a stack.
(mf_current): New static variable.
(step_data): New static function. This is where we do the
retention-resistant step of c->data.
(v_skip, v_fuzz, v_gather, v_collect, match_files_byref):
Use step_data function rather than c->data = rest(c->data).
(v_next_impl, extract): Register the new scanning context
as current by assigning it to mf_current, taking care
to save and restore the previous value, even if the
matching is abandoned by an exception.

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