Improve castNode notation by introducing list-extraction-specific variants.

This extends the castNode() notation introduced by commit 5bcab1114 to
provide, in one step, extraction of a list cell's pointer and coercion to
a concrete node type.  For example, "lfirst_node(Foo, lc)" is the same
as "castNode(Foo, lfirst(lc))".  Almost half of the uses of castNode
that have appeared so far include a list extraction call, so this is
pretty widely useful, and it saves a few more keystrokes compared to the
old way.

As with the previous patch, back-patch the addition of these macros to
pg_list.h, so that the notation will be available when back-patching.

Patch by me, after an idea of Andrew Gierth's.

Discussion: https://postgr.es/m/14197.1491841216@sss.pgh.pa.us

src/backend/utils/mmgr/portalmem.c

@@ -153,7 +153,7 @@ PortalGetPrimaryStmt(Portal portal)
 
 	foreach(lc, portal->stmts)
 	{
-		PlannedStmt *stmt = castNode(PlannedStmt, lfirst(lc));
+		PlannedStmt *stmt = lfirst_node(PlannedStmt, lc);
 
 		if (stmt->canSetTag)
 			return stmt;