[tsl:concurrency] NFC: Add a benchmark for AsyncValuePtr<T>::Map(executor, ...)
[tsl:concurrency] NFC: Add a benchmark for AsyncValuePtr<T>::Map(executor, ...)

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Benchmark                           Time             CPU   Iterations
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BM_MapIntToFloat                 38.3 ns         38.3 ns     18232592
BM_MapIntToFloatOnExecutor       53.1 ns         53.1 ns     13204614
