The `OlmMachine` constructor now has 2 more optional arguments:
`store_name` and `store_passphrase`, to use Indexed DB as the backend
to store the Olm machine keys, rather than having an in-memory Olm
machine.
Node.js is used to test our binding. Indexed DB is absent of
Node.js. So, firstly, we are using the `fake-indexeddb` JavaScript
library to mimic the same API inside Node.js. And, secondly, we use
our own fork of the `indexed_db_futures` Rust crate to provide add
support for Node.js too ([PR is
here](https://github.com/Alorel/rust-indexed-db/pull/11)). It
basically looks in the Node.js global environment if the `indexedDB`
getter is present, just like it is already done for `Window` on any
browser, or `WorkerGlobalScope` for workers.
Jonas Platte