nip77: convert to dealing with bytes instead of hex strings.

it was cool but this should be faster and less confusing.

nip77/negentropy/storage/empty/empty.go

@@ -23,6 +23,6 @@ func (Empty) GetBound(idx int) negentropy.Bound {
 	return negentropy.InfiniteBound
 }
 
-func (Empty) Fingerprint(begin, end int) string {
+func (Empty) Fingerprint(begin, end int) [negentropy.FingerprintSize]byte {
 	return acc.GetFingerprint(end - begin)
 }