Add examples that call a complex pre-compiled library

I'm interested in using `wasmtime` to call out to complex originally-built-in-C libraries from Python - things like `ffmpeg` or the various image compression libraries used by https://squoosh.app/ (e.g. https://squoosh.app/c/mozjpeg_enc-f6bf569c.wasm)

It would be really cool if the documentation or examples for wasmtime included an example of that kind of usage!