[PATCH 0/4]: C++ P1423R3 char8_t remediation implementation

[Tom Honermann]

This series of patches provides an implementation of the changes for C++ 
proposal P1423R3 [1].

These changes do not impact default libstdc++ behavior for C++17 and 
earlier; they are only active for C++2a or when the -fchar8_t option is 
specified.

Tested x86_64-linux.

Patch 1: Decouple constraints for u8path from path constructors.
Patch 2: Update __cpp_lib_char8_t feature test macro value, add deleted 
operators, update u8path.
Patch 3: Updates to existing tests.
Patch 4: New tests.

Tom.

[1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1423r3.html