Remove circular dependencies on aes crate

[tarcieri]

Several crates in this repo use the aes crate as a dev-dependency.

The aes crate lists ctr in its (hard) dependencies section now.

This means that upgrading the cipher crate right now involves at least a temporary breakage. Here are some example PRs where this has happened:

#224 #225 #226 #227

There are at least two different ways this could be addressed, I think:

1. Migrate the SIMD CTR implementation from the aes crate into the ctr crate, since it's not ctr-specific
2. Use a different "example" block cipher as a dev-dependency

[newpavlov]

I think a better approach would be to move all block mode crates to the block ciphers repository, as attempted in the RustCrypto/block-ciphers#233.