Change gist stratnum function to use CompareType

Change gist stratnum function to use CompareType

This changes commit 7406ab623fe in that the gist strategy number
mapping support function is changed to use the CompareType enum as
input, instead of the "well-known" RT*StrategyNumber strategy numbers.

This is a bit cleaner, since you are not dealing with two sets of
strategy numbers.  Also, this will enable us to subsume this system
into a more general system of using CompareType to define operator
semantics across index methods.

Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com