Hopefully fixes #221.

The ZIG variable helps to test ncdu with different zig installations,
and it allows Gentoo to inject the zig version that should be used to
build zig into the Makefile.

Also add a phony 'build' target as first target to the Makefile so
that it becomes the default target. This allows the Gentoo package to
use the default src_compile() function.

See also https://bugs.gentoo.org/900547

More visible than just bold.

This is useful for building binary distributions because it allows
references to library dependendencies on the build machine to be
rewritten appropriately upon installation on the user's machine.

Zig also does this in their `build.zig`:

    https://github.com/ziglang/zig/blob/b52be973dfb7d1408218b8e75800a2da3dc69108/build.zig#L551-L554

With a little help from IRC:

<ifreund> Ayo: its probaly stupidly copying that array to the stack to do the
          safety check, pretty sure there's an open issue on this still
<ifreund> you may be able to work around the compiler's stupidity by using a
          pointer to the array or slice or something
<Ayo> ifreund: Yup, (&self.rdbuf)[self.rdoff] does the trick, thanks.
<ifreund> no problem! should get fixed eventually

Interesting case of
https://ziglang.org/download/0.10.0/release-notes.html#Escaped-Pointer-to-Parameter

Still using a few embedded packed structs for those fields that benefit
from bit packing. This isn't much cleaner than using packed structs for
everything, but it does have better semantics. In particular, all fields
(except those inside nested packed structs) are now guaranteed to be
byte-aligned and I don't have to worry about the memory representation
of integers when pointer-casting between the different Entry types.

Building is currently broken on packed struct alignment issues. :/

Broken in 1548f927 because I'm an idiot.

Probably also fixes #210, but I don't have a Mac to test.

Zig requires alignment to be specified when specifying a fill character,
as otherwise digits specified after ':' are interpreted as part of the
field width.

The missing alignment specifier caused character codes < 0x10 to be
serialized incorrectly, producing an export file ncdu could not import.

For example, a character with code 1 would be serialized as '\u00 1'
instead of '\u0001'.

A directory of test files can be generated using:

    mkdir test_files; i=1; while [ $i -le 255 ]; do c="$(printf "$(printf "\\\\x%02xZ" "$i")")"; c="${c%Z}"; touch "test_files/$c"; i=$((i+1)); done

Behavioral changes:
- A single wildcard ('*') does not cross directory boundary anymore.
  Previously 'a*b' would also match 'a/b', but no other tool that I am
  aware of matches paths that way. This change breaks compatibility with
  old exclude patterns but improves consistency with other tools.
- Patterns with a trailing '/' now prevent recursing into the directory.
  Previously any directory excluded with such a pattern would show up as
  a regular directory with all its contents excluded, but now the
  directory entry itself shows up as excluded.
- If the path given to ncdu matches one of the exclude patterns, the old
  implementation would exclude every file/dir being read, this new
  implementation would instead ignore the rule. Not quite sure how to
  best handle this case, perhaps just exit with an error message?

Performance wise, I haven't yet found a scenario where this
implementation is slower than the old one and it's *significantly*
faster in some cases - in particular when using a large amount of
patterns, especially with literal paths and file names.

That's not to say this implementation is anywhere near optimal:
- A list of relevant patterns is constructed for each directory being
  scanned. It may be possible to merge pattern lists that share
  the same prefix, which could both reduce memory use and the number of
  patterns that need to be matched upon entering a directory.
- A hash table with dynamic arrays as values is just garbage from a
  memory allocation point of view.
- This still uses libc fnmatch(), but there's an opportunity to
  precompile patterns for faster matching.

The other files were already indirectly referenced, but it's good to
make it explicit.

Based on #204.

I wasn't planning on (publicly) keeping up with Zig master before the
next release, but it's looking like 0.10 will mainly focus on the new
stage2 compiler and there might not be any significant language/stdlib
changes. If that's the case, might as well pull in this little change in
order to increase chances of ncdu working out of the box when 0.10 is
out.

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

While it's true that the root item can't be a special, the first item to
be added is not necessarily the root item. In particular, it isn't when
refreshing.

Probably fixes #194

Because, even in 2022, there are systems where the libc locale is not,
in fact, UTF-8. Fixes #186.

Change "eigth-block" to "eighth-block"