Change the interface again, hopefully this time a bit more normal.
Now we wrap the emscripten module completely and just expose the
high level objects.

The olm library export is now imported as normal (ie. returns
a module rather than a function returning a module) but has an
`init` method which *must* be called. This returns a promise
which resolves when the module is ready. It also rejects if the
module failed to set up, unlike before (and unlike the
promise-not-a-promise that emscripten returns).

Generally catch failures to init the module.

The closure compiler was just renaming the variable so it never
would have picked them up. Make it an extern so it knows what to do.