chacha20: avoid panic on counter overflow

[tarcieri]

To avoid repeating the keystream on counter overflow, #68 added an assertion that the counter value is less than chacha20::MAX_BLOCKS.

Ideally SalsaFamilyCipher::block can be changed to use a fallible API, so instead of panicking it returns an error when the counter overflows.

[srijs]

Would it be worth thinking about how to achieve this w/o breaking changes (e.g. add a try_block method with a sensible default impl), or do you generally think this warrants a major version bump?

[tarcieri]

@srijs I'd like to make some potentially breaking changes anyway. I opened a separate thread about those: #71

[tarcieri]

I believe this logic should address the issue:

https://github.com/RustCrypto/stream-ciphers/blob/68e3467/chacha20/src/cipher.rs#L182-L188