And move all the rendering, ansi_term, colourful stuff into the output modules, which is the only place they should be used!

Exa now uses the new IO, Path, and Filesystem libraries that have been out for a while now.

Unfortunately, the new libraries don't *entirely* cover the range of the old libraries just yet: in particular, to become more cross-platform, the data in `UnstableFileStat` isn't available in the Unix `MetadataExt` yet. Much of this is contained in rust-lang/rfcs#1044 (which is due to be implemented in rust-lang/rust#14711), but it's not *entirely* there yet.

As such, this commits a serious loss of functionality: no symlink viewing, no hard links or blocks, or users or groups. Also, some of the code could now be optimised. I just wanted to commit this to sort out most of the 'teething problems' of having a different path system in advance.

Here's an example problem that took ages to fix for you, just because you read this far: when I first got exa to compile, it worked mostly fine, except calling `exa` by itself didn't list the current directory. I traced where the command-line options were being generated, to where files and directories were sorted, to where the threads were spawned... and the problem turned out to be that it was using the full path as the file name, rather than just the last component, and these paths happened to begin with `.`, so it thought they were dotfiles.

This module provides feature-specific implementations, and also dummy implementations for when they aren't supported by the system or OS.

Doing it this way limits all the #[cfg(feature)] annotations, as we can now just include the module or not.

- old_path::Path isn't imported by default anymore
- range -> ..

The challenge is that the paths returned from libgit2's status listing
are from the perspective of the Git repository and thus effectively
relative to the working tree root, while the other paths we're
manipulating are (potentially) relative to our current working
directory. So, if those two aren't identical (if running from outside
the working tree, or from a subdirectory), the paths won't match up.

A reasonably reliable way around this is to resolve both types of paths
to absolute paths before comparing them. This fixes #15 at a basic
level, anyway.

What still doesn't work: referring to the working tree or one of its
descendants via a symlink. For that, we'd probably need to fully resolve
symlinks in the file path.

(The unwrap_or()'s are messy and will probably just result in missing
status information, but then, what information could you hope to get
without having both a current working directory and a Git working tree?)

There's still a lot to do, but this is actually *something*. The tree hierarchy is displayed using hashes at the start of a line. I want to have it just before the filename, but this will need some changes to the way that columns are handled.

- 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.