Fix std::filesystem::path Database constructor (fixes #317)

[Chronial]

The return type of path.c_str() is platform-dependent and is a wstring on windows. path.u8string() always returns an utf8-encoded string.

In c++17, that string has type std::string, but starting in c++20 it is std::u8string. I used a reinterpret_cast to handle the c++20 case.

Alternatively, a constructor taking u8string could be added for the c++20 case. But it seems odd to have only one place where the library accepts u8strings. If support for u8strings is added, it should probably be done for the whole library at once.

[coveralls]

Coverage remained the same at 100.0% when pulling 42ac337 on Chronial:feature/fs_path into ef650e0 on SRombauts:master.

[chronoxor]

I confirm that the fix works well for VS2019, but not for MinGW64 8.1.0 due to - https://stackoverflow.com/questions/50890493/compile-error-when-stdfilesystem-header-file-is-added-to-my-program

I add more complex compiler checking:

// c++17: MinGW GCC version > 8
// c++17: Visual Studio 2017 version 15.7
#if ((__cplusplus >= 201703L) && ((!defined(__MINGW32__) && !defined(__MINGW64__)) || (__GNUC__ > 8))) || (defined(_MSVC_LANG) && (_MSVC_LANG >= 201703L))

[SRombauts]

Hello, I believe that this is not needed anymore now that I merged the other ones, right?
I'll close it but let me know if something is still incorrect

[Chronial]

Yes, this was already merged as part of #318