[tsl:concurrency] NFC: Add a benchmark for AsyncValuePtr<T>::Map
[tsl:concurrency] NFC: Add a benchmark for AsyncValuePtr<T>::Map

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Benchmark                 Time             CPU   Iterations
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BM_MapIntToFloat       38.4 ns         38.4 ns     18157872

All of the overheads are in AsyncValueRef<float> reference counting.
