psbt: save the index of the change on the 'parent'

Note that for removals, the index will be on the original; for
additions, the index will be on the new. Yes this is implicit.

common/psbt_open.c

@@ -219,6 +219,7 @@ void psbt_sort_by_serial_id(struct wally_psbt *psbt)
 		struct type##_set a;				\
 		a.type = from->type##s[index];			\
 		a.tx_##type = from->tx->type##s[index]; 	\
+		a.idx = index;					\
 		tal_arr_expand(&add_to, a);			\
 	} while (0)
 

common/psbt_open.h

@@ -19,11 +19,15 @@ struct wally_map;
 struct input_set {
 	struct wally_tx_input tx_input;
 	struct wally_psbt_input input;
+	/* index on PSBT of this input */
+	size_t idx;
 };
 
 struct output_set {
 	struct wally_tx_output tx_output;
 	struct wally_psbt_output output;
+	/* index on PSBT of this output */
+	size_t idx;
 };
 
 struct psbt_changeset {