Add stack depth checks to key recursive functions in backend/nodes/*.c.

Although copyfuncs.c has a check_stack_depth call in its recursion,
equalfuncs.c, outfuncs.c, and readfuncs.c lacked one.  This seems
unwise.

Likewise fix planstate_tree_walker(), in branches where that exists.

Discussion: https://postgr.es/m/30253.1544286631@sss.pgh.pa.us

src/backend/nodes/readfuncs.c

@@ -28,6 +28,7 @@
 
 #include <math.h>
 
+#include "miscadmin.h"
 #include "nodes/parsenodes.h"
 #include "nodes/readfuncs.h"
 
@@ -1382,6 +1383,9 @@ parseNodeString(void)
 
 	READ_TEMP_LOCALS();
 
+	/* Guard against stack overflow due to overly complex expressions */
+	check_stack_depth();
+
 	token = pg_strtok(&length);
 
 #define MATCH(tokname, namelen) \