Example should have performance numbers and comparisons

I'm doing some benchmarking, and while this might be apples to oranges,

1. Invoking quickjs as a wasmtime precompiled using `--eval console.log('hello world')` 1000 times takes approx 7-8s
2. Invoking the hello-world.js embedding in the example 1000 times takes approx 80s or maybe 10x as much time.

```
time for x in `seq 1 1000`; do wasmtime --allow-precompiled quickcheck.cwasm --eval "console.log('hello world')"; done

real 0m7.849s
user 0m3.077s
sys 0m6.752s
```

```
$ time for x in `seq 1 1000`; do ./target/release/wasmtime-test > /dev/null; done

real 1m21.672s
user 0m45.470s
sys 0m38.263s
```

Using `https://gitlab.com/api/v4/projects/47807501/packages/generic/js_interpreters/0.0.1/cweb_quickcheck.wasm` for the example.

It would be good to understand better what performance we can expect and how to benchmark against other options.