Move all the counter management to a submodule that can, in the future,
be encapsulated from the rest of the AES and AES-GCM logic.

`git difftool HEAD^1:src/aead/aes.rs src/aead/aes/counter.rs`

Move `Conuter` and `Iv` to aes/counter.rs.

Create a more robust internal API for counter/nonce/IV management that
makes the usage within AES-GCM more clearly correct. The new design is
easier to test.

`git difftool HEAD^1:src/aead/aes.rs src/aead/aes/counter.rs`