rmp_serde::to_writer

From quite a few data format used with serde have the same interface where I could use the same `to_writer` and I was surprised that msgpack-rust did differently (unexpected for me).

```rust
let mut buffer = File::create("test").expect("serialize");
serde_cbor::to_writer(&mut buffer, &data);
serde_json::to_writer(&mut buffer, &data);
```

Of course, I figured out later that I could:

```rust
rmp_serde::encode::write(&mut buffer, &data);
```

Is there any plan if the api will be generalized or any reason to use a different syntax even though each of them does the same?