Fix memory leaks with Combine subscriptions

[jgale]

I was experimenting with this awesome tutorial and realized the TaskCellViewModel was never getting deallocated.

I researched it and came across this helpful Swift form post: https://forums.swift.org/t/does-assign-to-produce-memory-leaks/29546/11

Adapted quote:

Indeed there is a retain cycle with the sample code that uses assign:

The TaskCellViewModel retains the AnyCancellable (1), which retains the Subscribers.Assign subscription (2), which retains the Bar (3) in order to be able to perform the assignment. Cycle completed: the TaskCellViewModel is never deinited.

I fixed this by:

1. Using [weak self] for the Combine subscriptions that use sink
2. Added a extension on Publisher that makes assign not retain the object, which was from the above forum post

If you prefer not overloading the assign name I'm happy to rename it assignNoRetain or something like that for clarity.

After this change, a deinit() on TaskCellViewModel does get regularly called. I should note that I still see leaks when running this new code in Instruments, but it's a little beyond me to understand why. 😞

[peterfriese]

Thanks for finding this issue and sending a fix, Jeremy!