further explain changes

Per recent discussion on pgsql-performance, and per discussion on
-hackers that it might not be too late for small patches after all,
here is a patch (as yet without documentation) which adds some
additional instrumentation to EXPLAIN for hashes: number of buckets,
number of batches, original number of batches, and peak memory
utilization. Thoughts?

I was also thinking about the possibility of adding a new option
called "output" and making that control whether the "Output" line gets
printed. It's kind of annoying to use EXPLAIN (ANALYZE, VERBOSE)
right now (and moreso with this patch) specifically because of that
line, which is quite... verbose. If we're going to change it ever we
should do it for 9.0, since we've made a lot of other changes that
people will be adjusting for anyhow.

Also, do we want to change the schema URL? The existing URL was
suggested by Peter but IIRC there was some thought that it might not
be the best choice.

http://www.postgresql.org/2009/explain

...Robert