psbt: sort maps before serializing, to stabilize comparison

Our psbt input/output comparison functions use serialization to compare
the things, but if there's a map with things in it and the map isn't
sorted exactly the same, it's highly likely you'll mark an identical inputs
as different.

To fix this, we sort all the input/output maps before linearizing them.

common/psbt_open.c

@@ -68,9 +68,12 @@ static const u8 *linearize_input(const tal_t *ctx,
 		abort();
 	psbt->inputs[0] = *in;
 	psbt->num_inputs++;
-	/* Blank out unknowns. These are unordered and serializing
-	 * them might have different outputs for identical data */
-	psbt->inputs[0].unknowns.num_items = 0;
+
+
+	/* Sort the inputs, so serializing them is ok */
+	wally_map_sort(&psbt->inputs[0].unknowns, 0);
+	wally_map_sort(&psbt->inputs[0].keypaths, 0);
+	wally_map_sort(&psbt->inputs[0].signatures, 0);
 
 	const u8 *bytes = psbt_get_bytes(ctx, psbt, &byte_len);
 
@@ -97,9 +100,9 @@ static const u8 *linearize_output(const tal_t *ctx,
 
 	psbt->outputs[0] = *out;
 	psbt->num_outputs++;
-	/* Blank out unknowns. These are unordered and serializing
-	 * them might have different outputs for identical data */
-	psbt->outputs[0].unknowns.num_items = 0;
+	/* Sort the outputs, so serializing them is ok */
+	wally_map_sort(&psbt->outputs[0].unknowns, 0);
+	wally_map_sort(&psbt->outputs[0].keypaths, 0);
 
 	const u8 *bytes = psbt_get_bytes(ctx, psbt, &byte_len);