Add btree_gin support for enum types

Reviewed by Tom Lane and Anastasia Lubennikova

Discussion:  http://postgr.es/m/56EA8A71.8060107@dunslane.net

contrib/btree_gin/expected/enum.out

@@ -0,0 +1,63 @@
+set enable_seqscan=off;
+CREATE TYPE rainbow AS ENUM ('r','o','y','g','b','i','v');
+CREATE TABLE test_enum (
+   i rainbow
+);
+INSERT INTO test_enum VALUES ('v'),('y'),('r'),('g'),('o'),('i'),('b');
+CREATE INDEX idx_enum ON test_enum USING gin (i);
+SELECT * FROM test_enum WHERE i<'g'::rainbow ORDER BY i;
+ i 
+---
+ r
+ o
+ y
+(3 rows)
+
+SELECT * FROM test_enum WHERE i<='g'::rainbow ORDER BY i;
+ i 
+---
+ r
+ o
+ y
+ g
+(4 rows)
+
+SELECT * FROM test_enum WHERE i='g'::rainbow ORDER BY i;
+ i 
+---
+ g
+(1 row)
+
+SELECT * FROM test_enum WHERE i>='g'::rainbow ORDER BY i;
+ i 
+---
+ g
+ b
+ i
+ v
+(4 rows)
+
+SELECT * FROM test_enum WHERE i>'g'::rainbow ORDER BY i;
+ i 
+---
+ b
+ i
+ v
+(3 rows)
+
+explain (costs off) SELECT * FROM test_enum WHERE i>='g'::rainbow ORDER BY i;
+                  QUERY PLAN                   
+-----------------------------------------------
+ Sort
+   Sort Key: i
+   ->  Bitmap Heap Scan on test_enum
+         Recheck Cond: (i >= 'g'::rainbow)
+         ->  Bitmap Index Scan on idx_enum
+               Index Cond: (i >= 'g'::rainbow)
+(6 rows)
+
+-- make sure we handle the non-evenly-numbered oid case for enums
+create type e as enum ('0', '2', '3');
+alter type e add value '1' after '0';
+create table t as select (i % 4)::text::e from generate_series(0, 100000) as i;
+create index on t using gin (e);