Identity columns

This is the SQL standard-conforming variant of PostgreSQL's serial
columns.  It fixes a few usability issues that serial columns have:

- CREATE TABLE / LIKE copies default but refers to same sequence
- cannot add/drop serialness with ALTER TABLE
- dropping default does not drop sequence
- need to grant separate privileges to sequence
- other slight weirdnesses because serial is some kind of special macro

Reviewed-by: Vitaly Burovoy <vitaly.burovoy@gmail.com>

src/include/executor/execExpr.h

@@ -144,6 +144,7 @@ typedef enum ExprEvalOp
 	EEOP_NULLIF,
 	EEOP_SQLVALUEFUNCTION,
 	EEOP_CURRENTOFEXPR,
+	EEOP_NEXTVALUEEXPR,
 	EEOP_ARRAYEXPR,
 	EEOP_ARRAYCOERCE,
 	EEOP_ROW,
@@ -361,6 +362,13 @@ typedef struct ExprEvalStep
 			SQLValueFunction *svf;
 		}			sqlvaluefunction;
 
+		/* for EEOP_NEXTVALUEXPR */
+		struct
+		{
+			Oid			seqid;
+			Oid			seqtypid;
+		}			nextvalueexpr;
+
 		/* for EEOP_ARRAYEXPR */
 		struct
 		{