Reimplement final_act using the scope_exit design from P0052R5

[CaseyCarter]

Fixes #125.

[neilmacintosh]

Thanks for the PR @CaseyCarter!

At the moment, we like the simplicity of the design of final_act as it stands (modulo bugs). We can revisit whether to harmonize final_act with a standardized facility that offers similar functionality once there is such a thing (P0052 is still in-flight for now).

So I'll leave thi PR open, but mark it "deferred" for now (because I wanted to use that label and have never had a chance before ;-)).

[JordanMaples]

Maintainer's call: closing this as the code has diverged significantly. Additionally, it looks like most of the changes @CaseyCarter wanted to implement were added in other PRs since this. Please file an issue or re-open if there are additional improvements you'd like to add to final_action. Thanks!