Make Labels selection not O(n^2)

[Luthaf]

Instead we can exploit the underlying HashMap structure of Labels.

Fixes #319, bringing the calculation down to 2.5s on my machine. There might be more optimizations to do here, but at least the worst issue is gone.

╔════╦══════════════════════════════════════════════╦════════════╦═══════════╦══════════╦══════════╗
║ id ║ span name                                    ║ call count ║ called by ║ total    ║ mean     ║
╠════╬══════════════════════════════════════════════╬════════════╬═══════════╬══════════╬══════════╣
║  1 ║ Calculator::prepare                          ║          1 ║         — ║    1.65s ║    1.65s ║
╠════╬══════════════════════════════════════════════╬════════════╬═══════════╬══════════╬══════════╣
║  0 ║ NeighborsList                                ║          1 ║         1 ║ 375.89ms ║ 375.89ms ║
╠════╬══════════════════════════════════════════════╬════════════╬═══════════╬══════════╬══════════╣
║  2 ║ NeighborList::compute                        ║          1 ║         — ║ 786.14ms ║ 786.14ms ║
╚════╩══════════════════════════════════════════════╩════════════╩═══════════╩══════════╩══════════╝

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📚 Documentation preview 📚: https://rascaline--320.org.readthedocs.build/en/320/

[github-actions[bot]]

Here is a pre-built version of the code in this pull request: wheels.zip, you can install it locally by unzipping wheels.zip and using pip to install the file matching your system