Improve WASM size

[olanod]

The compiled file in pkg/wasmer_wasi_js_bg.wasm being 609KB feels a bit heavy for a something that is basically a wrapper of a built-in WASM engine. Would be nice to see ideas on how this could be improved, I guess it requires going all the way up the dependency chain doing optimizations that might not be trivial but should be worth the effort :)