Add memo cache to improve performance of repeated parse calls

[phenomnomnominal]

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I've got some code that repeatedly calls esquery.query() with the same selector. Obviously this can be quite slow, but instead of rewriting it all I figured it might be worth just caching internally. Keen for thoughts.

[phenomnomnominal]

As an aside, since tsquery uses esquery.parse , tsquery will get faster too!