cipher: merge block-modes crate

[newpavlov]

@o-klp has kindly shared with us ownership of the cipher crate.

I think it could be reasonable to merge traits from the listed crates into a single crate, this would reduce size of dependency tree a bit in some cases and will allow us to naturally introduce a trait (and support types) covering all kinds of non-authenticating ciphers.

[tarcieri]

Thanks @o-klp!

I unified the block-cipher and stream-cipher crates into cipher in #337, and released the result as cipher v0.2.0 in #337.

I'll leave this issue open to track merging block-modes into cipher.