This PR splits `BlockBackend` traits into 4 specific traits:
`BlockCipherEncBackend`, `BlockCipherDecBackend`, `BlockModeEncBackend`,
and `BlockModeDecBackend`. Same for `BlockClosure`. This allows for
cipher backends to remove awkard `&mut &backend` juggling (see
https://github.com/RustCrypto/block-ciphers/pull/442), makes code a bit
easier to read (i.e. `encrypt_blocks` instead of `proc_blocks`), and
allows for one backend type to be used for both encryption and
decryption.

The `impl_simple_block_encdec` macro is removed since we now can
implement the backend traits directly on cipher types, which should make
implementation crates slightly easier to understand.

Additionally, it moves traits to the `block` and `cipher` modules to
reduce clutter in the crate root. Later we can add docs to each module
to describe the traits in detail.