Add transaction to event slots

[fcollonval]

Fixes #232 Fixes #234

Add transaction to event slots.

This ends up a bit more complex to be able to keep a reference to the transaction in the event, this PR introduced a BaseTransaction that only keeps reference to the document and the origin (independently of the rust object). That ensures the introspection of the event at any time.

It also introduces a more robust way to maintain the mapping of Python origin and rust origin.

[davidbrochart]

Thanks for the PR, I will have a look soon.

[davidbrochart]

@fcollonval Could you explain the reasoning about adding a reference count to origins?

[davidbrochart]

Not using a counter will result in discarding the origin from the mapping too early.

I don't think it's the case if the transaction is dropped after removing the origin from the doc.

[davidbrochart]

Closing as no feedback, feel free to reopen.