Update StringKeyed... benchmarks to tell the whole story
Update StringKeyed... benchmarks to tell the whole story

Summary:
Benchmark each type with its native key type and with its non-native key type.
This allows you to make a more-informed decision about which to use based on
the types you have available at insertion/lookup time.

Reviewed By: yfeldblum

Differential Revision: D4826527

fbshipit-source-id: ee4ef5255d2bdd955e9deb30dc4c28be183cee07

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