std::filesystem::path support

[f-michaut]

Adding std::filesystem::path support in cereal.

Relying on the current std::string serialization since it is the underling storage for filesystem::path.

[AzothAmmo]

This looks pretty good, it needs a unit test like the other types, and that unit test should be properly gated behind C++17 conditionals like similar features.

[f-michaut]

Hi there, sorry for the small delay on this PR lol

I have added the required tests, however it's failing for XMLInputArchive and JSONInputArchive and I dont understand why.

Error message :

error: static assertion failed due to requirement 'traits::detail::count_input_serializers<std::filesystem::path, cereal::JSONInputArchive>::value != 0': cereal could not find any input serialization functions for the provided type and archive combination.

Types must either have a serialize function, load/save pair, or load_minimal/save_minimal pair (you may not mix these). 

Which I'm not sure I follow, because obviously I defined both save/load pair and its working fine for the Binary archives, and furthermore I looked at the save/load functions of the std::string type and they are identical to mine.

However the string type seems to be able to run tests on Json + XML archives without a problem...

Could you help me figure out why its not working ?

[f-michaut]

Also, small note that there was a few compilations errors in files unrelated to mines, but I fixed them regardless (in details/helpers.hpp and types/tuple.hpp).

You might want to double-check theses fixes, just to be sure. Tests are all passing, so I'm assuming its fine.