Changing the browser.worker API

[PierreQuentel]

In issue #2135, Ben Levitt reported that in the current implementation, stdlib modules are not cached in indexedDB, resulting in uselessly long loading times.

To support this feature, since indexedDB is asynchronous, loading a web worker from Brython must also be asynchronous.

While keeping the current version with the Worker class for backwards compatibility, I propose to add a new function create_worker(worker_id, onready, onmessage, onerror) that loads the Web Worker identified by worker_id and

• runs function onready when the indexedDB cache is loaded
• runs onmessage every time the worker sends a message
• runs onerror in case an error occurs in the worker

More detail in the module documentation.

Any comment / suggestion on this new API is welcome.