wasm Olm has a new interface: it now has an init method that needs
to be called and the promise it returns waited on before the Olm
module is used. Support that, and allow Crypto etc to be imported
whether Olm is enabled or not. Change whether olm is enabled to
be async since now it will be unavailable if the async module init
fails. Don't call getOlmVersion() until the Olm.init() is done.

By doing `Olm = global.Olm` on script load, we require that Olm is
available right from the start, which isn't really necessary. As
long as it appears some time before we actually want to use it,
this is fine (we can probably assume it's not going to go away
again..?)

This means Riot doesn't need to faff about making sure olm is
loaded before starting anything else.