Move all computation off-the-mainthread

This already the case on the web, or at-least it's moved the browser which is most likely moving it off-the-mainthread.

We should do the same on Android and iOS.

A workaround is to wrap functions using crypto with [compute](https://api.flutter.dev/flutter/foundation/compute.html).

**NOTE**, the API is already fully async, so we should be able to move computation off the main thread, without breaking existing users of this package.