Code review for protransform patches.

Fix loss of previous expression-simplification work when a transform
function fires: we must not simply revert to untransformed input tree.
Instead build a dummy FuncExpr node to pass to the transform function.
This has the additional advantage of providing a simpler, more uniform
API for transform functions.

Move documentation to a somewhat less buried spot, relocate some
poorly-placed code, be more wary of null constants and invalid typmod
values, add an opr_sanity check on protransform function signatures,
and some other minor cosmetic adjustments.

Note: although this patch touches pg_proc.h, no need for catversion
bump, because the changes are cosmetic and don't actually change the
intended catalog contents.

src/include/nodes/nodeFuncs.h

@@ -29,6 +29,7 @@
 extern Oid	exprType(const Node *expr);
 extern int32 exprTypmod(const Node *expr);
 extern bool exprIsLengthCoercion(const Node *expr, int32 *coercedTypmod);
+extern Node *relabel_to_typmod(Node *expr, int32 typmod);
 extern bool expression_returns_set(Node *clause);
 
 extern Oid	exprCollation(const Node *expr);