This commit moves file, dir, and the feature modules into one parent 'fs' module. Now there are three main 'areas' of the code: main and options, the filesystem-touching code, and the output-displaying code.

It should be the case that nothing in 'output' touches 'std::fs'.

Fixes #108. MetadataExt now returns direct numeric types rather than platform-specific ones, so we need to adjust the functions that use these to have the new types. I've just aliased the types to specific ones so the rest of the code remains the same (file.rs is the only place that uses this)

The RFC that changed this is here: https://github.com/rust-lang/rust/pull/31551

This commit changes all the views to accommodate printing each path's prefix, if it has one.

Previously, each file was stripped of its ancestry, leaving only its file name to be displayed. So running "exa /usr/bin/*" would display only filenames, while running "ls /usr/bin/*" would display each file prefixed with "/usr/bin/". But running "ls /usr/bin/" -- without the glob -- would run ls on just the directory, printing out the file names with no prefix or anything.

This functionality turned out to be useful in quite a few situations: firstly, if the user passes in files from different directories, it would be hard to tell where they came from (especially if they have the same name, such as find | xargs). Secondly, this also applied when following symlinks, making it unclear exactly which file a symlink would be pointing to.

The reason that it did it this way beforehand was that I didn't think of these use-cases, rather than for any technical reason; this new method should not have any drawbacks save making the output slightly wider in a few cases. Compatibility with ls is also a big plus.

Fixes #104, and relates to #88 and #92.

This fixes a bug where extra sorting options (dirs first, reverse) were not applied when listing in long mode. In other words, fixes #105.

The bug occurred because the sorting function only took Files, but the details view uses File eggs that only contain Files. This commit changes the sorting function to accept anything that AsRefs to File, and impls that on both File and Egg so the same function works for both.

This limits it to stable until the APIs stabilise (weird, huh?)

Thinking about it, it doesn't make sense to use an *external* time zone source when the program we want to compare it to, ls, uses the system one. So just use the system one.

Also, handle the case where the time zone data file can't be loaded by showing the files in UTC rather than falling over and quitting.

This finishes off and closes #102. The code in this and the previous commit
were partly authored by `zv0n` on GitHub.

This uses the case-insensitive sort function in the `natord` crate to
convert both strings to lowercase lazily, sorting them as it goes. It
also adds tests for `--sort` in general.

The case sensitivity has been made an enum so it can be reused for other
fields (say, the file extension).

See #102.

- Fix visibility errors I stupidly didn't test before committing earlier
  today
- Silence warnings about casting that were necessary for ARM
- Update dependencies

Update Cargo dependencies

Travis didn't like the old versions.

It wasn't the Arc unwrap causing the crash on Linux. Maybe it's this.

Something about these seemed to be causing a crash on Travis (build 327)... I have no idea what would set it off, but this makes the code better anyway.

- Users v0.5.1, which renames OSUsers to UsersCache
- Locale v0.2, which returns to libc v0.1
- Datetime v0.4.2, which mimics the locale update, and puts timezone definitions in:
- Zoneinfo-data, which is needed to obtain the current timezone

The `--long` flag should show the '@' character in the permissions list if that feature has been compiled in, but only the `--extended` flag should actually show their keys, rather than just their presence.

This makes use of a change in the `users` crate to change which parts of exa's code are accessed under a `Mutex`. The change is that the methods on `Users` can now take just `&self`, instead of `&mut self`. This has a knock-on effect in exa, as many methods now don't need to take a mutable `&self`, meaning that the Mutex can be moved to only containing the users information instead of having to be queried for *every column*. This means that threading should now be a lot faster, as fewer parts have to be executed on a single thread.

The main change to facilitate this is that `Table`'s structure has changed: everything environmental that gets loaded at the beginning is now in an `Environment` struct, which can be mocked out if necessary, as one of `Table`'s fields. (They were kind of in a variety of places before.)

Casualties include having to make some of the test code more verbose, as it explicitly takes the columns and environment as references rather than values, and those both need to be put on the stack beforehand. Also, all the colours are now hidden behind an `opts` field, so a lot of the rendering code is more verbose too (but not greatly so).

Document using `cargo install` in README

Cargo now lets people install binaries by running `cargo install`. This
can be more convenient than cloning and building exa manually.

This commit separates the code used to generate the tree structure characters from the code used to build tables, meaning that it'll become possible to display tree structures without using any of the table code.

Also, some tests are added to make sure that the tree code *basically* works.

1. imports from std
2. imports from external crates
3. imports from local modules
4. imports from self

This commit moves the colours module to be a sub-module of the output one.
This makes sense because finding which colour a certain file should be is only
done during output, and (I think) the only places that the `Colours` struct's
fields are ever queried is from the output module.

The only casualty was that the `file_colour` from the filetype module had to
be moved, as determining colours is no longer part of that module - only
determining filetype is. So it now reflects its name!

The benefit of this is that it make it possible to convert text cell contents
vectors into text cells with a method (see next commit). Casualties include
having to call `.into()` on vectors everywhere, which I'm not convinced is a
bad thing.