Compress prost! messages with zstd, optional tokio channels support

Simple compress/decompress

```rust fn doroundtripcoders(dummies: Vec) { let mut encoder = ProstEncoder::new(0).unwrap(); for dummy in &dummies { encoder.write(dummy).unwrap(); } let compressed = encoder.finish().unwrap();

let mut decoder =
    ProstDecoder::<proto::Dummy>::new_decompressed(compressed.as_slice()).unwrap();

let mut i: usize = 0;
while let Some(dummy) = decoder.next() {
    let dummy = dummy.unwrap();
    assert_eq!(&dummy, dummies.get(i).unwrap());
    i += 1;
}

assert_eq!(dummies.len(), i);

} ```

Tokio channels support

Cargo.toml:

toml prosto = { version = "0.1", features = ["enable-tokio"] }

```rust fn doroundtripchannels(dummies: Vec) { tracingsubscriber::fmt::tryinit().ok();

let mut rt = Runtime::new().unwrap();

let (mut source, urx) = mpsc::channel::<proto::Dummy>(dummies.len());
let (ctx, crx) = mpsc::channel::<Vec<u8>>(dummies.len());
let (utx, mut sink) = mpsc::channel::<proto::Dummy>(dummies.len());

let compressor = Compressor::new(urx, ctx, 256 * 1024, 5);
let decompressor = Decompressor::new(crx, utx);

rt.block_on(async move {
    tokio::task::spawn(compressor.compress());
    tokio::task::spawn(decompressor.decompress());

    for dummy in &dummies {
        source.send(dummy.clone()).await.unwrap();
    }

    std::mem::drop(source);

    let mut i: usize = 0;
    while let Some(dummy) = sink.recv().await {
        assert_eq!(&dummy, dummies.get(i).unwrap());
        i += 1;
    }

    assert_eq!(dummies.len(), i);
});

} ```