Allow aggregate transition states to be serialized and deserialized.

This is necessary infrastructure for supporting parallel aggregation
for aggregates whose transition type is "internal".  Such values
can't be passed between cooperating processes, because they are
just pointers.

David Rowley, reviewed by Tomas Vondra and by me.

src/include/parser/parse_agg.h

@@ -51,6 +51,12 @@ extern void build_aggregate_combinefn_expr(Oid agg_state_type,
 										   Oid combinefn_oid,
 										   Expr **combinefnexpr);
 
+extern void build_aggregate_serialfn_expr(Oid agg_state_type,
+										  Oid agg_serial_type,
+										  Oid agg_input_collation,
+										  Oid serialfn_oid,
+										  Expr **serialfnexpr);
+
 extern void build_aggregate_finalfn_expr(Oid *agg_input_types,
 						int num_finalfn_inputs,
 						Oid agg_state_type,