Promise chaining the emscripten module `then` function breaks it badly,
causing an infinite loop.
I'm unsure about the source of the issue, but most likely at this point
is due to the old emscripten version (I remember very old html5 builds
having issue with promise chaining too).
With this commit, we no longer use the module as a promise, and
instantiate it using `Promise` objects directly for compatibility.
ae3c9345cc