New path API

As previously discussed with @privat and @R4PaSs, this is a propostion for a new file access API. 
It provides new features:

* All file/path related services are to be stored in `Path` instead of `String`. The goal is to store there all the methods previously beginning with `path_` and others like `rmdir` and `files`.  The calls to the new API take the form of `"/tmp/somefile.txt".to_path.delete`. You will notice that this PR do not yet move all those services but only a subset as an example, this will be completed if it is appreciated.
* Easy file opening (for read or write) with: `"/tmp/somefile.txt".to_path.open_ro.read_all`
* `Path` stores the concept of full path and filename. Calls to `/etc/somedir/".to_path.files` returns an array of `FilePath` which can be `to_s`ed to get the full path and calling `filename` returns only the file name.
* The native `stat` structure is now renamed to `NativeFileStat` and protected within a Nity finalizable `FileStat`. The new `FileStat` could be implemented differently by platform.