rust-index: improve phase computation speed While less memory efficient, using an array is *much* faster than using a HashMap, especially with the default hasher. It even makes the code simpler, so I'm not really sure what I was thinking in the first place, maybe it's more obvious now. This fix a significant performance regression when using the rust version of the code. (however, the C code still outperform rust on this operation) hg perf::phases on mozilla-try-2023-03-22 - 6.6.3: 0.451239 seconds - before: 0.982495 seconds - after: 0.265347 seconds - C code: 0.183241 second

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