Consolidate crypto functionality in `ethcore-crypto`. (#8432)

* Consolidate crypto functionality in `ethcore-crypto`.

- Move `ecdh`/`ecies` modules to `ethkey`.
- Refactor `ethcore-crypto` to use file per module.
- Replace `subtle` with `ethcore_crypto::is_equal`.
- Add `aes_gcm` module to `ethcore-crypto`.

* Rename `aes::{encrypt,decrypt,decrypt_cbc}` ...

... to `aes::{encrypt_128_ctr,decrypt_128_ctr,decrypt_128_cbc}`.