result: add result assignment extenstion point for `?`

[arnetheduck]

When ? exits early, it does so using variations return v - however, in frameworks such as chronos, we need to change return v to future.complete(v) - this feature allows chronos to inject a template that performs this assignment instead of using the "ordinary" flow.

Risk:

assignResult might already be taken as a name, and multiple frameworks might compete for the functionality.

Potential alternative:

Instead of assignResult, this construct could be called returnResult, and it would be responsible for breaking the control flow (performing the return in addition to assigning the result).

Other interactions:

https://github.com/status-im/nim-stew/pull/34 proposes a general-purpose conversion framework - this could be used to automatically convert the error based on existing conversions - the use case is that oftentimes, one error needs to be converted to another in a call chain and injecting a converter simplifies this flow - this might however create an problematic interaction with an assignResult template in the future.

[arnetheduck]

marking draft for now - failing in 2.0

[arnetheduck]

obsoleted by https://github.com/arnetheduck/nim-results/pull/37