Use ByteArray instead of ToByteArray() for HashNode.ToBencodex() implementation

[greymistcube]

Usage of ToByteArray() results in copying the whole byte[] backing the ImmutableArray<byte> inside a HashDigest<SHA256>, once when calling HashDigest<SHA256>.ToByteArray() and once more when calling new Binary(byte[]). This results in an unnecessary usage of resources, both memory and cpu.