Zero-overhead native Go support

Currently there's a local proxy `Up Proxy -> Your App`, however with native Go we could now let you run native Go apps without the proxy, by embedding it, something like:

```
func main() {
up.Handle(h)
}
```

Honestly I think the latency benefits would be negligible, but it's worth investigating. Another benefit would be that you can shave ~3mb off your deploy size (and time).

TL;DR this keeps all of the Up proxy features, with none of the overhead.

- [ ] remove `Runtime`? decide how to signal native support
- [ ] add nicer public interface to `"github.com/apex/up/platform/lambda/proxy/golang"`
- [ ] add config to opt-in and/or detect (too much magic probably haha)
- [ ] add examples
- [ ] ensure shim is still used by default
- [ ] ensure `up start` respects this
- [ ] QA
- [ ] docs (mention missing relay features for logs etc)