pgsql: Improve caching in range type I/O functions.

Improve caching in range type I/O functions.

Cache the the element type's I/O info across calls, not only the range
type's info. In passing, also clean up hash_range a bit more.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/04da3232907680caad3445928c97a246c626a14a

Modified Files
--------------
src/backend/utils/adt/rangetypes.c | 316 ++++++++++++++++++++----------------
1 files changed, 176 insertions(+), 140 deletions(-)