C++20 compatible polyfill for <expected>

[Bronek]

ONLY A DUMMY AT THIS TIME

I plan to add actual implementation and unit tests, honest. The goal is to remove a major blocker for C++20 compatibility.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.7%. Comparing base (5205895) to head (4ef3afd).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #155     +/-   ##
=======================================
+ Coverage   99.5%   99.7%   +0.1%     
=======================================
  Files         19      20      +1     
  Lines        827     862     +35     
  Branches     121     121             
=======================================
+ Hits         823     859     +36     
+ Misses         4       3      -1     

Files with missing lines	Coverage Δ
include/fn/and_then.hpp	100.0% <ø> (ø)
include/fn/choice.hpp	100.0% <ø> (+2.0%)	⬆️
include/fn/detail/functional.hpp	100.0% <ø> (ø)
include/fn/detail/pack_impl.hpp	100.0% <ø> (ø)
include/fn/detail/variadic_union.hpp	98.7% <ø> (ø)
include/fn/expected.hpp	99.7% <ø> (ø)
include/fn/fail.hpp	100.0% <ø> (ø)
include/fn/filter.hpp	100.0% <ø> (ø)
include/fn/functional.hpp	100.0% <ø> (ø)
include/fn/inspect.hpp	100.0% <ø> (ø)
... and 10 more