Gist and Gin support for enums

Recently I submitted a patch (too late for release 9.6, unfortunately) to add support for enum data types to the btree_gist and btree_gin additional modules. This came out of a desire to be able to use an enum field in an exclusion constraint. That really only requires GiST support, but since I was adding enum support to btree_gist it seemed a good idea to add it to btree_gin as well, so that, for example, enum fields can be used in multi-column GIN indexes.

Here's an exclusion constraint example using enums, adapted from the Postgres docs.

andrew=# create extension btree_gist;
CREATE EXTENSION
andrew=# \dx
                           List of installed extensions
    Name    | Version |   Schema   |                  Description                  
------------+---------+------------+-----------------------------------------------
 btree_gist | 1.2     | public     | support for indexing common datatypes in GiST
 plpgsql    | 1.0     | pg_catalog | PL/pgSQL procedural language
(2 rows)

andrew=# create type animal as enum ('zebra','lion');
CREATE TYPE
andrew=# create table zoo (cage int, exhibit animal, exclude using gist(cage with =, exhibit with <>));
CREATE TABLE
andrew=# INSERT INTO zoo VALUES(123, 'zebra');
INSERT 0 1
andrew=# INSERT INTO zoo VALUES(123, 'zebra');
INSERT 0 1
andrew=# INSERT INTO zoo VALUES(123, 'lion');
ERROR:  conflicting key value violates exclusion constraint "zoo_cage_exhibit_excl"
DETAIL:  Key (cage, exhibit)=(123, lion) conflicts with existing key (cage, exhibit)=(123, zebra).
andrew=#