remark about the interface for chacha20

[UnixJunkie]

Hello,
In Stream.chacha20, I think that the (currently optional) [ctr] argument should be made mandatory.
I feel it is dangerous for users to have both the [iv] and [ctr] have default values.
I don't vote for the [iv] to be mandatory, because of its type it is difficult to increment.
While, for the counter, we can ensure it is unique by always incrementing it (until it loops).
Regards,
F.