Add a Fixed(u8) constructor to Colour, which represents the 256 colours
that some terminals support. This means we can:

- stop using black bold to mean grey, which looks weird on terminals
  that haven't been set up to use it;
- support a *lot* more file type colours.

I'm a little suspicious of how much string allocation is being done in
colours.rs, but that's a problem for another time.

I think this is necessary because 'bytes' currently has no 'B' suffix, and it's
kind of hard to distinguish a long number from a suffix.

Currently there's only one numeric column, and that's the file size, so it gets
special treatment.

I was originally going to have a folder file size field be filled up with '-'s
as far as it could go, leaving it entirely up to the column how its field gets
formatted. But then I saw just one '-' working just fine, so I left it like
that. In the first try, columns could do anything they want when padding a
string (including changing the padding character or just changing it entirely),
but now there's no point.

- s/to_owned/to_string/g
- TotalEq/TotalOrd changes
- snake_case of identifiers

This finally means that the list of columns is no longer fixed, which means it has to be generated somewhere. And guess where it got moved to? That's right, the options object! (see previous commits)

I'd like to have as few public methods on it as possible.

The code could do with some comments, but it works.

I'm copying ls here because we don't really need the 'B' for bytes to be
listed every time. I think it looks better the new way. Unlike ls, don't
list directory sizes, because I've never found the pseudo-sizes they get
given at all useful.

Also, fix a bug where aligning columns didn't work when the number of
format characters (like '\x1B' and '[') were different between each
line.

They get computed often enough for this to be worthwhile, and I have plans for using the extension even if it's not what's being sorted by.

And move options struct into its own module

See https://github.com/mozilla/rust/pull/14310

This involves putting the entire output into a table before anything
is actually printed, in order to determine what the width of each
column should be. This should make it appear to output slower, as the
first line can only be printed after every file has been examined, but
it's still fast to me.

This meant writing some libc interaction, which wasn't actually too bad.

Replace ~strs with either &'static strs or .to_owned() strs where appropriate
Fields now seem to be private by default - good!

Also, reverse the way columns are rendered: before, a column took a stat and a name to render; now, a file takes a column type to render. This means that most of the File data/methods can be private.

Also add command-line-option-handling code.

Thanks to #rust for helping me diagnose lifetime problems!