psbt: add 'wally_psbt_clone' function, to clone a psbt

We'll need this for settng the tx correctly, for reasons

bitcoin/psbt.c

@@ -17,6 +17,18 @@
 	memmove((arr) + (pos), (arr) + (pos) + 1,		\
 		sizeof(*(arr)) * ((num) - ((pos) + 1)))
 
+/* FIXME: someday this will break, because it's been exposed in libwally */
+int wally_psbt_clone(const struct wally_psbt *psbt, struct wally_psbt **output)
+{
+	int ret;
+	size_t byte_len;
+	const u8 *bytes = psbt_get_bytes(NULL, psbt, &byte_len);
+
+	ret = wally_psbt_from_bytes(bytes, byte_len, output);
+	tal_free(bytes);
+	return ret;
+}
+
 void psbt_destroy(struct wally_psbt *psbt)
 {
 	wally_psbt_free(psbt);

bitcoin/psbt.h

@@ -12,6 +12,8 @@ struct wally_psbt_input;
 struct wally_tx;
 struct amount_sat;
 
+int wally_psbt_clone(const struct wally_psbt *psbt, struct wally_psbt **output);
+
 void psbt_destroy(struct wally_psbt *psbt);
 
 struct wally_psbt *new_psbt(const tal_t *ctx,