Ext_path
combine path1 path2
1. add some simplifications when concatenating 2. when path2
is absolute, return path2
get_extension "a.txt" = ".txt"
get_extension "a" = ""
TODO: could be highly optimized if from
and to
resolve to the same path, a zero-length string is returned Given that two paths are directory
A typical use case is
Filename.concat
(rel_normalized_absolute_path cwd (Filename.dirname a))
(Filename.basename a)