show Heap Fetches in EXPLAIN for index-only scans

I think that people who are using index-only scans are going to want
some way to find out the degree to which the scans are in fact
index-only.

So here's a 5-line patch that adds the number of heap fetches to the
EXPLAIN ANALYZE output.  This might not be all the instrumentation
we'll ever want here, but I think we at least want this much.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company