Update `rc4` (et alii?) to use `cipher` 0.5

The latest version of the `rc4` crate (and possibly other crates in this repository) uses `cipher` v0.4.3, while the latest version of `cipher` is v0.5.1.  This is an impediment to interoperability with the latest versions of other RustCrypto crates.  Please release an updated version soon.