This PR addresses the potential for memory leakage with the previous caching scheme; it should also be more efficient, resulting in fewer instances where a fresh Babel config must be generated. Documentation has been added, and `overrides` have been removed in favor of conditional plugins for consistency.
This approach is easier to break, in that `CharacteristicsGermaneToCaching` must be correct for all cases. However, it means that <= 8 configs will need to be generated within the context of a single thread, and the config-caching is about as fast as we can get it.
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