Remove std templates from interface

I would remove std::string (and other templates) from the interface and would replace it with const char* when returning for example the file name, etc. I believe this might be problematic if one tries to link a debug library with the release version of the cppfs

No source files or tests are named. Start by locating the public C++ interface and reviewing where std::string or other templates cross the library boundary, especially returned file names. Done means the proposed interface change is defined consistently and the debug/release linking concern is addressed.

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