Add boost::unordered_flat_map to the benchmark

[atyuwen]

Boost 1.81 released its own unordered_flat_map implementation, it will be appreciated if you could add that to the benchmark as well.

[ktprime]

from my benchmark( https://github.com/ktprime/emhash/blob/master/bench/martinus_bench.cpp ), unordered_flat_map seems to be the fastest in most cases.