Add a way to prevent the Module from being exported globally.

In our application, we stuff emscripten into a closure in pre- and post-
JS files, and export our interface without exposing the Module that we
use to implement some of it.
In `tests/test_other.py`. Basically build with and without this option, and do some checks on the generated code and on the result of executing it. See the last few tests in the file, should show the general form.