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/optimizer/util/clauses.c

@@ -3090,7 +3090,7 @@ eval_const_expressions_mutator(Node *node,
 				const_true_cond = false;
 				foreach(arg, caseexpr->args)
 				{
-					CaseWhen   *oldcasewhen = castNode(CaseWhen, lfirst(arg));
+					CaseWhen   *oldcasewhen = lfirst_node(CaseWhen, arg);
 					Node	   *casecond;
 					Node	   *caseresult;