This will cause a nice speedup if the temporary directory happens to be on
the same file system as the target directory for the BAT Pack. This can be
influenced by setting an environment variable; see the documentation for
[tempfile.gettempdir](https://docs.python.org/3/library/tempfile.html#tempfile.gettempdir)
This does introduce some not-so-nice things, like having to annotate each
`__init__` function with `-> None`. However, the benefits of having static
type checking in a complex bit of software like BAT outweigh the downsides.
- Tracer now iterates over blocks in disk order.
- Packer copies files per directory, in a separate thread.
- Packer only copies files if they don't exist yet.
- Packer also copies file permissions.
No path rewriting yet, just simply copying the dependencies. They are
copied to the target directory + the path relative to the current working
directory. For example:
bat pack scenes/05-barber/05_02_C-agent_focused/05_02_C.lighting.blend /tmp/packer
will write to
/tmp/packer/scenes/05-barber/05_02_C-agent_focused/05_02_C.lighting.blend
and will copy ./path/to/file → /tmp/packer/path/to/file
