This was my favourite feature, so I'm glad to see it back!

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.

Still missing a few Beta features, but it compiles!

- Copy requires Clone
- current_dir returns a Path now
- num_cpus moved to a crate

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 path_filename function/method isn't declared public. If the declaration is changed, rustc complains about a conflict. This seems to be the minimal change needed for tests to run.

current_dir returns a new PathBuf, but getcwd returns an old Path. One day I'll have to upgrade to the new Path library. One day.

Fixes #41.

Closes #27.

In cases where symlink targets were more than a single directory down,
exa did not print the '/' targets when separating directories, resulting
in the following output:

    symlink => dirAdirBdirC/file

Instead of

    symlink => dirA/dirB/dirC/file

By adding a '/' character after each component of the filename, this
error is fixed.

Fixes #26.

The old implementation blindly assumed that a symlink target would have a directory compoment, which the current directory, parent directory, and root directory technically don't have.

Fixes #20.

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?)

- Feature flags changes (old_io, old_path)
- Slice syntax changes ([] -> [..])

- Update package versions
- locale::default -> locale::english
- Argument handling changes

This has been mostly done with changes in the datetime crate's suddenly
supporting locales.

It's still important that the user's locale is touched only once and
cached from that point on, so a struct in output::details has been made
public, along with that module. This will change later as that object
gains more and more uses thoughout the codes.

Also, fix the broken tests by just using the default locale at test
time.

Use the `locale` crate as a dependency to read in the set
thousands-separator character, and pass this to the file size column,
which uses it to add the separators in.

en_GB uses ","
fr_FR uses "" and just displays the numbers in one go.

Otherwise, just display the hour and minute.

Using the datetime crate, add an extra column to the --long view that
prints out the modified, accessed, or created timestamp for each file.
Also, let the user pick which one they want to see based on the --time
command-line option.

- Turn the views and main program loop into structs, rather than just as one gigantic function
- Separate views into their own files

The addition of the git column and the tree view meant that a lot of functions now just took extra arguments that didn't seem to fit. For example, it didn't really work to have only one 'view' method that printed out everything, as the different view options now all take different parameters.