Convert strategies to and from compare types

Convert strategies to and from compare types

For each Index AM, provide a mapping between operator strategies and
the system-wide generic concept of a comparison type.  For example,
for btree, BTLessStrategyNumber maps to and from COMPARE_LT.  Numerous
places in the planner and executor think directly in terms of btree
strategy numbers (and a few in terms of hash strategy numbers.)  These
should be converted over subsequent commits to think in terms of
CompareType instead.  (This commit doesn't make any use of this API
yet.)

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com