fixed rsa2 to work with binary strings; added fuzz test

[whisk]

Description

rsa2 implementation failed to encrypt/decrypt binary strings because it was encoding runes, 1 or 2 byte sequences, packed into 64 bits; but it was always decoding 64 bits as a full rune

Fix

• fixed rsa2 implementation to work with binary strings by using byte-to-byte processing
• minor performance improvement by using strings.Builder instead of appending a string
• added a fuzz test as suggested in Add FuzzXxx tests to cipher algorithms #480