Implementing multiple worlds

[jsturtevant]

It doesn't seem possible today to register multiple worlds. For instance, if I wanted my app to support both wasi:cli and wasi:http.

I thought it might be possible to create two different modules and then register them like:

    let rt = cli::bindings::register_host_functions(&mut sb, state);
    let rt = http::bindings::register_host_functions(&mut rt, state);

This doesn't work due to export WIT_WORLD which must be set to one and will only generate guest bindings for one of them.

The work around for now is combine the worlds into one world.