5142: analysis-stats: allow parallel type inference r=matklad a=jonas-schievink
This is mostly just for testing/fun, but it looks like type inference can be sped up massively with little to no effort (since it runs after the serial phases are already done).
Without `--parallel`:
```
Item Collection: 16.43597698s, 683mb allocated 720mb resident
Inference: 25.429774879s, 1720mb allocated 1781mb resident
Total: 41.865866352s, 1720mb allocated 1781mb resident
```
With `--parallel`:
```
Item Collection: 16.380369815s, 683mb allocated 735mb resident
Parallel Inference: 7.449166445s, 1721mb allocated 1812mb resident
Inference: 143.437157ms, 1721mb allocated 1812mb resident
Total: 23.973303611s, 1721mb allocated 1812mb resident
```
Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>
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