cipher: revert SeekNum::from_block_byte change

[tarcieri]

This reverts the implementation of SeekNum::from_block_byte which was merged as part of #435.

I'm not exactly sure what the issue is and it's somewhat difficult to debug in that it's code that involves both generics and macros causing an error as part of any failure in a long chain of checked arithmetic, which I'm trying to debug from the context of a concrete stream cipher impl (both chacha20 and salsa20) where it's operating over a generic type.

The error manifests as OverflowError:

RustCrypto/stream-ciphers#205 (comment)

This commit reverts to the previous implementation, which is at least much simpler.

[tarcieri]

I've confirmed this fixes the test failures in the chacha20 and salsa20 crates locally.

[tarcieri]

@newpavlov this appears to be a bug in the new implementation of this logic, although I'm not sure what specifically the problem was

[newpavlov]

I will try to look into it.

[newpavlov]

Ah, I've changed how block-buffer handles counters to simplify downstream implementations. Let's say we have two counters (block_counter, buffer_counter), assuming we process one byte at a time previously we had sequence (0, 0), (0, 1), (0, 2), etc. In the new version we have the following sequence instead: (0, 0), (1, 1), (1, 2), etc. It allows to merge block counter increment and generation of a new block.

[tarcieri]

@newpavlov is there something that needs to change in chacha20 and salsa20? I was confused why their test suites were failing with OverflowError:

RustCrypto/stream-ciphers#205 (comment)

[newpavlov]

Probably, yes. I will need some time to refresh my memory of how they structured. When working on the cipher PR i was mostly working with the ctr crate, so I am not sure how compatible the introduced changes with chacha20 and other stream ciphers.

[tarcieri]

Both chacha20 and salsa20 contain what was originally copypasta from the ctr crate, so it likely needs to be updated in both.

Hopefully they can lean more heavily on block-buffer going forward to simplify these updates.