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/executor/nodeModifyTable.c

@@ -1968,7 +1968,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	 */
 	foreach(l, node->rowMarks)
 	{
-		PlanRowMark *rc = castNode(PlanRowMark, lfirst(l));
+		PlanRowMark *rc = lfirst_node(PlanRowMark, l);
 		ExecRowMark *erm;
 
 		/* ignore "parent" rowmarks; they are irrelevant at runtime */