Add transforms feature

This provides a mechanism for specifying conversions between SQL data
types and procedural languages.  As examples, there are transforms
for hstore and ltree for PL/Perl and PL/Python.

reviews by Pavel Stěhule and Andres Freund

src/backend/tcop/utility.c

@@ -174,6 +174,7 @@ check_xact_readonly(Node *parsetree)
 		case T_CreateTableAsStmt:
 		case T_RefreshMatViewStmt:
 		case T_CreateTableSpaceStmt:
+		case T_CreateTransformStmt:
 		case T_CreateTrigStmt:
 		case T_CompositeTypeStmt:
 		case T_CreateEnumStmt:
@@ -1314,6 +1315,10 @@ ProcessUtilitySlow(Node *parsetree,
 				DefineOpFamily((CreateOpFamilyStmt *) parsetree);
 				break;
 
+			case T_CreateTransformStmt:
+				CreateTransform((CreateTransformStmt *) parsetree);
+				break;
+
 			case T_AlterOpFamilyStmt:
 				AlterOpFamily((AlterOpFamilyStmt *) parsetree);
 				break;
@@ -2004,6 +2009,9 @@ CreateCommandTag(Node *parsetree)
 				case OBJECT_POLICY:
 					tag = "DROP POLICY";
 					break;
+				case OBJECT_TRANSFORM:
+					tag = "DROP TRANSFORM";
+					break;
 				default:
 					tag = "???";
 			}
@@ -2263,6 +2271,10 @@ CreateCommandTag(Node *parsetree)
 			}
 			break;
 
+		case T_CreateTransformStmt:
+			tag = "CREATE TRANSFORM";
+			break;
+
 		case T_CreateTrigStmt:
 			tag = "CREATE TRIGGER";
 			break;
@@ -2888,6 +2900,10 @@ GetCommandLogLevel(Node *parsetree)
 			lev = LOGSTMT_DDL;
 			break;
 
+		case T_CreateTransformStmt:
+			lev = LOGSTMT_DDL;
+			break;
+
 		case T_AlterOpFamilyStmt:
 			lev = LOGSTMT_DDL;
 			break;