fix: address leftover open bugs

[slapec93]

• Bytes copies its input, so changing the caller's array later no longer changes the object.
• base32 pads to a multiple of 8, as RFC 4648 specifies (it used to pad to 4)
• hexToUint8Array throws on invalid or odd-length hex
• rsEncode([]) throws a clear error instead of a TypeError
• xorCypher throws on an empty key instead of returning the input unchanged
• Stamper rejects a batch depth that isn't a whole number above 16
• indexOf returns start for an empty needle instead of -1