Fixes #199.
That's not to say it handles variation selectors or combining marks
well, though. This is kind of messy. :(

Fixes #183

Same thing as commit 376aad0d in the C
version.

Sticking to "compiletime-known" error types will essentially just bring
in *every* possible error anyway, so might as well take advantage of
@errorName.

This complicated the scan code more than I had anticipated and has a
few inherent bugs with respect to calculating shared hardlink sizes.

Still, the merge approach avoids creating a full copy of the subtree, so
that's another memory usage related win compared to the C version.
On the other hand, it does leak memory if nodes can't be reused.

Not quite as well tested as I should have, so I'm sure there's bugs.

Two differences compared to the C version:
- You can now select individual paths in the listing, pressing enter
  will open the selected path in the browser window.
- Creating this listing is much slower and requires, in the worst case,
  a full traversal through the in-memory tree. I've tested this without
  the same-dev and shared-parent optimizations (i.e. worst case) on an
  import with 30M files and performance was still quite acceptable - the
  listing completed in a second - so I didn't bother adding a loading
  indicator. On slower systems and even larger trees this may be a
  little annoying, though.

(also, calling nonl() apparently breaks detection of the return key,
neither \n nor KEY_ENTER are emitted for some reason)

Doesn't display the item's path anymore (seems rather redundant) but
adds a few more other fields.

The good news is: apart from this little thing, everything seems to just
work(tm) on FreeBSD. Think I had more trouble with C because of minor
header file differences.

In a similar way to the C version of ncdu: by wrapping malloc(). It's
simpler to handle allocation failures at the source to allow for easy
retries, pushing the retries up the stack will complicate code somewhat
more. Likewise, this is a best-effort approach to handling OOM,
allocation failures in ncurses aren't handled and display glitches may
occur when we get an OOM inside a drawing function.

This is a somewhat un-Zig-like way of handling errors and adds
scary-looking 'catch unreachable's all over the code, but that's okay.

Performance is looking great, but the code is rather ugly and
potentially buggy. Also doesn't handle hard links without an "nlink"
field yet.

Error handling of the import code is different from what I've been doing
until now. That's intentional, I'll change error handling of other
pieces to call ui.die() directly rather than propagating error enums.
The approach is less testable but conceptually simpler, it's perfectly
fine for a tiny application like ncdu.

I plan to add more display options, but ran out of keys to bind.
Probably going for a quick-select menu thingy so that we can keep the
old key bindings for people accustomed to it.

The graph width algorithm is slightly different, but I think this one's
a minor improvement.

The exported file format is fully compatible with ncdu 1.x, but has a
few minor differences. I've backported these changes in
ca51d4ed

I initially wanted to keep a directory's block count and size as a
separate field so that exporting an in-memory tree to a JSON dump would
be easier to do, but that doesn't seem like a common operation to
optimize for. We'll probably need the algorithms to subtract sub-items
from directory counts anyway, so such an export can still be
implemented, albeit slower.

libc locale-dependent APIs are pure madness, but I can't avoid them as
long as I use ncurses. libtickit seems like a much saner alternative (at
first glance), but no popular application seems to use it. :(

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.