Use more unique yet short paths for logging

[purplesyringa]

Now ZeroNet logs tend to be literally useless for debugging because most of the paths are incomprehensible. I propose replacing the path shortening algorithm with a more subtle one. Here are the major changes:

1. Paths such as src/Ui/__init__.py, should they appear in a traceback, are now encoded as Ui/__init__.py (was: __init__.py).
2. Paths such as plugins/Chart/ChartDb.py are now encoded as ChartDb.py (was: Chart/ChartDb.py). In general, paths such as A/AB/ABC/X.py are now encoded as ABC/X.py to remove unnecessary prefixes.
3. Paths such as <frozen importlib._bootstrap>, which may appear in a traceback due to import errors, are now skipped (was: <frozen importlib._bootstrap>).
4. As the algorithm has changed from 'only keep specific prefixes' (ex.: plugin name) to 'only remove unnecessary prefixes' (ex.: .../src/), paths outside ZeroNet root, should they appear in a traceback, are now anonymized: /root/ZeroNet/src/main.py is now encoded as ?/src/main.py (was: main.py).
5. Third-party module paths now include module name: .../site-packages/gevent/__init__.py is now encoded as <gevent>/__init__.py (was: __init__.py).
6. Builtin python modules are now visibly separated from ZeroNet files: /usr/lib/python3.8/os.py is now encoded as <os> (was: os.py).
7. Gevent paths such as src/gevent/event.py are correctly parsed as <gevent>/event.py (was: event.py).

I've also added some tests.

[HelloZeroNet]

Thanks, looks better this way, added some fix for Windows path separators: ecfb6b6