Rewritten Index Advisor patch

Awhile back I complained that I didn't like the way that the index advisor
patch plugged into the system:
http://archives.postgresql.org/pgsql-hackers/2007-04/msg00346.php

Attached is a proposed replacement patch that keeps essentially all the
advisor logic outside the core backend, and uses the method I suggested of
modifying the result of get_relation_info() rather than installing phony
system-catalog entries. Most of the patch bulk is actually just a small
refactoring of ExplainOnePlan's API to make it more convenient to call
from an advisor plugin. I also added hooks to let an advisor work through
EXPLAIN, as I still maintain is a more useful behavior than doubling the
work involved in every planner call. However, the planner() hook is still
there for those who are insistent.

To test the code, I made up a silly little proof-of-concept "advisor" that
just checks to see if 2-column indexes would be more helpful if the column
order were switched. It's incomplete because I didn't do anything about
printing out a nice explanation of what the hypothetical index is.

Comments, objections?

regards, tom lane