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

This extends the castNode() notation introduced by commit 5bcab11 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

Author: tglsfdc

src/include/nodes/pg_list.h

@@ -106,26 +106,32 @@ list_length(const List *l)
 #define lfirst(lc)				((lc)->data.ptr_value)
 #define lfirst_int(lc)			((lc)->data.int_value)
 #define lfirst_oid(lc)			((lc)->data.oid_value)
+#define lfirst_node(type,lc)	castNode(type, lfirst(lc))
 
 #define linitial(l)				lfirst(list_head(l))
 #define linitial_int(l)			lfirst_int(list_head(l))
 #define linitial_oid(l)			lfirst_oid(list_head(l))
+#define linitial_node(type,l)	castNode(type, linitial(l))
 
 #define lsecond(l)				lfirst(lnext(list_head(l)))
 #define lsecond_int(l)			lfirst_int(lnext(list_head(l)))
 #define lsecond_oid(l)			lfirst_oid(lnext(list_head(l)))
+#define lsecond_node(type,l)	castNode(type, lsecond(l))
 
 #define lthird(l)				lfirst(lnext(lnext(list_head(l))))
 #define lthird_int(l)			lfirst_int(lnext(lnext(list_head(l))))
 #define lthird_oid(l)			lfirst_oid(lnext(lnext(list_head(l))))
+#define lthird_node(type,l)		castNode(type, lthird(l))
 
 #define lfourth(l)				lfirst(lnext(lnext(lnext(list_head(l)))))
 #define lfourth_int(l)			lfirst_int(lnext(lnext(lnext(list_head(l)))))
 #define lfourth_oid(l)			lfirst_oid(lnext(lnext(lnext(list_head(l)))))
+#define lfourth_node(type,l)	castNode(type, lfourth(l))
 
 #define llast(l)				lfirst(list_tail(l))
 #define llast_int(l)			lfirst_int(list_tail(l))
 #define llast_oid(l)			lfirst_oid(list_tail(l))
+#define llast_node(type,l)		castNode(type, llast(l))
 
 /*
  * Convenience macros for building fixed-length lists
@@ -204,6 +210,7 @@ extern ListCell *list_nth_cell(const List *list, int n);
 extern void *list_nth(const List *list, int n);
 extern int	list_nth_int(const List *list, int n);
 extern Oid	list_nth_oid(const List *list, int n);
+#define list_nth_node(type,list,n)	castNode(type, list_nth(list, n))
 
 extern bool list_member(const List *list, const void *datum);
 extern bool list_member_ptr(const List *list, const void *datum);