Another day, another interface
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.