Moved is_path_separator() function and path_separator as a constexpr to the header file
Added path_separators string (for Windows, which includes \ and /)
get_canonical_path() works only when the file exists (returns an empty string in other case)
normalize_path() and get_absolute_path() work in a lexical level, handling only strings and getting the current path when needed, but without checking the existence of the file
Several refactors in this commit:
Fix #98