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/commands/aggregatecmds.c

@@ -109,13 +109,13 @@ DefineAggregate(ParseState *pstate, List *name, List *args, bool oldstyle, List
 			aggKind = AGGKIND_ORDERED_SET;
 		else
 			numDirectArgs = 0;
-		args = castNode(List, linitial(args));
+		args = linitial_node(List, args);
 	}
 
 	/* Examine aggregate's definition clauses */
 	foreach(pl, parameters)
 	{
-		DefElem    *defel = castNode(DefElem, lfirst(pl));
+		DefElem    *defel = lfirst_node(DefElem, pl);
 
 		/*
 		 * sfunc1, stype1, and initcond1 are accepted as obsolete spellings