Profiling showed that string parsing was a bottleneck. We rarely need
the full power of JSON strings, though, so we can optimize for the
common case of plain strings without escape codes. Keeping the slower
string parser as fallback, of course.
Previous import code did not correctly handle a non-empty directory with
the "read_error" flag set. I have no clue if that can ever happen in
practice, but at least ncdu 1.x can theoretically emit such JSON so we
handle it now.
Also fixes mtime display of "special" files. i.e. don't display the
mtime of the parent directory - that's confusing.
Split a generic-ish JSON parser out of the import code for easier
reasoning and implemented a few more performance improvements as well.
New code is ~30% faster in both ReleaseSafe and ReleaseFast.
