Functions returning structs are not working in no-modules mode

[ngg]

Smallest example that reproduces this bug:

#[wasm_bindgen]
pub struct S;

#[wasm_bindgen]
pub fn create_s() -> S {
    S
}

The generated JS file (with --no-modules) contains something like this:

__exports.create_s = function() {
    return S.__construct(wasm.create_s());
}

__exports.S = class S {
    // ...
}

When I call the create_s function it fails with ReferenceError: S is not defined.
If I modify the JS file either to call __exports.S.__construct or export the class as let S = null; __exports.S = S = class S { ... } then it works as supposed.

The second way seems like a really easy fix, I'll create a pull request.