lib: add String::/ for path jonction

Signed-off-by: Jean Privat <jean@pryen.org>
lib: add String::relpath for advanced path manipulation

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: String services on paths

Since @xymus added `%` on String, I add `/` !

I also implemented `relpath` that is really usefull (I need it for the compiler) but nitcorn and nitiwiki may also use it for their operations on relative URL or relative wiki-links

Pull-Request: #776
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>