/a/b/c/d/../../../e  ->  /a/e
                /a/../b/./c//d  ->  /b/c/d
      ../invalid_do_not_change  ->  ../invalid_do_not_change
                         ./tmp  ->  tmp
             /tmp/../../../tmp  ->  /tmp
                          A//B  ->  A/B
                          A/B/  ->  A/B
                         A/./B  ->  A/B
                    A/foo/../B  ->  A/B