cork/uncork in the encoder

in my code I have something like this:

```
something.pipe(lps.encode()).pipe(socket);
```

I think it will make sense to cork/uncork the target stream for the two pushes or push a concat'ed buffer:

```
this.push(pool.slice(used - varint.encode.bytes, used))
this.push(data)
```

Similar issue in another similar library:

https://github.com/rkusa/frame-stream/issues/4

Start in the encoder at the two this.push calls shown in the issue, then read the linked frame-stream issue for context on corking or combining output. Done means choosing and implementing the buffering approach while preserving the length-prefixed stream output.

Written by the indexing model from the issue text.