Make `strong_typedef` structural (#126)
Make `strong_typedef` structural (#126)

Switching `value_` from `private` to `public` make `strong_typdef` structural
and thus usable in Class Non-Type Template Parameter context in C++20 and
above. Fixes #125.

Co-authored-by: Luke D'Alessandro <58863626153781e4d679599700e5fc08d89850d2@uw.edu>
foonathan/type_safe,foonathan/type_safe