Clean up rewriter routines to use expression_tree_walker and

expression_tree_mutator rather than ad-hoc tree walking code.  This shortens
the code materially and fixes a fair number of sins of omission.  Also,
change modifyAggrefQual to *not* recurse into subselects, since its mission
is satisfied if it removes aggregate functions from the top level of a
WHERE clause.  This cures problems with queries of the form SELECT ...
WHERE x IN (SELECT ... HAVING something-using-an-aggregate), which would
formerly get mucked up by modifyAggrefQual.  The routine is still
fundamentally broken, of course, but I don't think there's any way to get
rid of it before we implement subselects in FROM ...