stdlib_system: essential path functionality

[wassup05]

A few path related functions for ease of future functionality have been added.

• joinpath: joins the given paths according to the platform's path-separator.
• operator(/): as was suggested in the Fortran discourse here an operator is also provided for the same functionality
• splitpath: splits the path following the last path-separator and returns the head and tail.
• basename: just returns the tail of splitpath
• dirname: just returns the head of splitpath

The pathsep parameter contains either / or \ depending on the platform and is a compile-time constant now, so is the parameter, ISWIN

Do let me know your thoughts.