Implement dynamic amount of tokens for change (#223)

With the recent update to NUT-08, we can ensure that the amount of blank
outputs is always enough to cover any overpaid lightning fees. This
change implements this functionality for both the wallet and the mint.
The mint updateis backwards-compatible with respect to old wallets.

cashu/core/helpers.py

@@ -1,4 +1,5 @@
 import asyncio
+import math
 from functools import partial, wraps
 from typing import List
 
@@ -42,3 +43,13 @@ def fee_reserve(amount_msat: int, internal=False) -> int:
         int(settings.lightning_reserve_fee_min),
         int(amount_msat * settings.lightning_fee_percent / 100.0),
     )
+
+
+def calculate_number_of_blank_outputs(fee_reserve_sat: int):
+    """Calculates the number of blank outputs used for returning overpaid fees.
+
+    The formula ensures that any overpaid fees can be represented by the blank outputs,
+    see NUT-08 for details.
+    """
+    assert fee_reserve_sat > 0, "Fee reserve has to be positive."
+    return max(math.ceil(math.log2(fee_reserve_sat)), 1)