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 99.5%. Comparing base (5205895) to head (f94cb5f).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #155   +/-   ##
=====================================
  Coverage   99.5%   99.5%           
=====================================
  Files         19      20    +1     
  Lines        827     841   +14     
  Branches     121     121           
=====================================
+ Hits         823     837   +14     
  Misses         4       4           

Files with missing lines	Coverage Δ
include/fn/and_then.hpp	100.0% <ø> (ø)
include/fn/choice.hpp	98.0% <ø> (ø)
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