Python-like abspath for c++

Python's os.path.abspath or Path.absolute are great: you give them a path, which might not exist, and you get a path you can use regardless of the current directory. os.path.abspath will also normalize it, while Path will not by default because with Path s a normal form is less needed. This is great to normalize input, regardless of if it's an existing file you're needing to open, or a new file…