[m-rev.] renaming parsing/unparsing modules

[Zoltan Somogyi]

As I discussed with Mark earlier today, I would like to rename
the unparse.m module he just created, because there are several
other modules that do unparsing: parse_tree_out*.m, prog_out.m,
and mercury_to_mercury.m. Since all the predicates now in unparse.m
convert parse tree components to terms, and none of parse_tree_out*.m
do, I propose parse_tree_to_term.m as the new name.

I would also like to rename those prog_io_*.m modules which do parsing
(not all of them do) to names such as parse_goal.m, parse_dcg_goal.m,
and so on.

Opinions?

Zoltan.