Support index-only scans using the visibility map to avoid heap fetches.

When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page.  This patch depends
on the previous patches that made the visibility map reliable.

There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.

Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.