[Feature Request] Implement proper behavior for `cancelled`, `uncancel`, and `cancelling` of activities and child workflows

[cretz]

Describe the solution you'd like

Implement proper behavior for cancelled, uncancel, and cancelling of activities and child workflows. The behavior probably looks like:

• cancelled - return true once it comes back as canceled
• uncancel - raise an (non-Temporal) exception saying that uncancel is not supported
• cancelling - probably just works already