Add vect_erase, VECT_ERASE.  VECT_POPBACK needs a dtor

vect_popback and VECT_POPBACK are now defined in terms of vect_erase and
VECT_ERASE, respectively.
diff --git a/lens_default.c b/lens_default.c
index 9f60dae..26ba22e 100644
--- a/lens_default.c
+++ b/lens_default.c
@@ -321,7 +321,7 @@
 	value_destroy(&element);
 
 done:
-	vect_popback(&pointers);
+	VECT_POPBACK(&pointers, struct value *, NULL, NULL);
 	return o;
 }