txr: deal with spurious retention problem.

When we open a data source using @(next), or use one
implicitly at the top level of the script, we would like
the top level scanning construct as @(collect) or @(repeat)
which does not backtrack not to consume memmory when moving
through a large file.

I experimented with ways to fix this in the past that
were ineffective, but I think I hit upon a working approach.

* match.c (match_files_ctx): Make the data field (pointer to
lazy list data source) volatile.
(match_files_byref): New function, based on converting
match_files to take a context by pointer rather than by value.
(match_files): By-value wrapper for match_files_byref that
most constructs use.
(v_next_impl): When opening a stream source, use
match_files_byref to avoid possible duplication of the
structure. Without the volatile in match_files_ctx, this
doesn't squelch all spurious retention.

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