- Rename std::io to std::old_io

This is good for two reasons: 1) shorter code! 2) it won't fail if any of the filenames aren't valid UTF-8.

This is something that I've long wanted to add. It uses libgit2 as an optional dependency.

- Prefer iter over into_iter where appropriate
- Cut down on cloning

This finally fixes the issue where trying to list a file causes a crash. Also, tidy up some of the uses of references.

This requires the dir field to store a reference to the path that
created it, so that path can be used to join onto the filename so it can
be properly statted.

Dir::readdir and File::from path now both return IoResults, rather than just calling fail! and exiting. This allows the program to continue after an error.

This new object is then passed down to the File, so it can see what files are around it. This means it doesn't need to re-query the filesystem several times per file, instead using the in-memory copy.

Also, switch to using the built-in Path#with_extension method.