Add enum const generation

- Make postgres name its enums
- Add way to filter columns by whether or not they're an enum
- Split parsing of enums into name & values
- Add strmangle check functions: IsEnumNormal, ShouldTitleCaseEnum
- Add new strmangle enum functions to template test
- Implement a set type called "once" inside the templates so that we can
  ensure certain things only generate one time via some unique name.

bdb/drivers/postgres.go

@@ -130,7 +130,7 @@ func (p *PostgresDriver) Columns(schema, tableName string) ([]bdb.Column, error)
 			case when c.data_type = 'USER-DEFINED' and c.udt_name <> 'hstore'
 			then
 			(
-				select 'enum(''' || string_agg(labels.label, ''',''') || ''')'
+				select 'enum.' || c.udt_name || '(''' || string_agg(labels.label, ''',''') || ''')'
 				from (
 					select pg_enum.enumlabel as label
 					from pg_enum