Eaiser to implement now that we're linking against libc.

But exclude pattern matching is extremely slow, so that should really be
rewritten with a custom fnmatch implementation. It's exactly as slow as
in ncdu 1.x as well, I'm surprised nobody's complained about it yet.
And while I'm at it, supporting .gitignore-style patterns would be
pretty neat, too.

I tried playing with zbox (pure Zig termbox-like lib) for a bit, but I
don't think I want to have to deal with the terminal support issues that
will inevitably come with it. I already stumbled upon one myself: it
doesn't properly put the terminal in a sensible state after cleanup in
tmux. As much as I dislike ncurses, it /is/ ubiquitous and tends to kind
of work.

The new data model is supposed to solve a few problems with ncdu 1.x's
'struct dir':
- Reduce memory overhead,
- Fix extremely slow counting of hard links in some scenarios
  (issue #121)
- Add support for counting 'shared' data with other directories
  (issue #36)

Quick memory usage comparison of my root directory with ~3.5 million
files (normal / extended mode):

  ncdu 1.15.1:     379M / 451M
  new (unaligned): 145M / 178M
  new (aligned):   155M / 200M

There's still a /lot/ of to-do's left before this is usable, however,
and there's a bunch of issues I haven't really decided on yet, such as
which TUI library to use.

Backporting this data model to the C version of ncdu is also possible,
but somewhat painful. Let's first see how far I get with Zig.