Jaded-Encoding-Thaumaturgy / vs-jetpack / 13621807102

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Use Vapoursynth's `register_on_destroy` to avoid caches leaking memory (#28)

I discovered there was a significant memory leak occurring, and traced
it back to the caches in `vs-tools`. The issue was that, because the
`Node`s stored in the caches keep references to the core, the
`finalize_core` hook meant to clear the caches was never being called,
because the caches themselves were keeping references to the core alive
and preventing `weakref.finalize` from running.

This change uses Vapoursynth's built-in `register_on_destroy` callback,
which hooks into the `Environment` instead of the `Core`, and causes the
destructors to run correctly and the caches to clear.

Minimal test script:
```python
import vapoursynth as vs
core = vs.core

import vstools
clip = core.bs.VideoSource(source="testfile.mkv")
matrix = vstools.Matrix.from_video(clip)
clip.set_output(0)
```

Valgrind Before (`vspipe -i test.vpy`):
```
==113446== LEAK SUMMARY:
==113446==    definitely lost: 0 bytes in 0 blocks
==113446==    indirectly lost: 0 bytes in 0 blocks
==113446==      possibly lost: 35,055,210 bytes in 349 blocks
==113446==    still reachable: 141,604,241 bytes in 10,666 blocks
==113446==         suppressed: 72 bytes in 3 blocks
```

Valgrind After:
```
==113200== LEAK SUMMARY:
==113200==    definitely lost: 73,192 bytes in 9 blocks
==113200==    indirectly lost: 0 bytes in 0 blocks
==113200==      possibly lost: 7,648 bytes in 7 blocks
==113200==    still reachable: 5,947,622 bytes in 5,489 blocks
==113200==         suppressed: 48 bytes in 1 blocks
```

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