Benchmark is implemented using Google Benchmark lib. Here's benchmark
settings:
 - values: we use structure (tuple) containing pointer to heap memory
           and size (all payload is of the same size - 32 bytes);
 - keys: unsigned char (first byte in the tuple memory);
 - hash function: FNV-1a;
 - value comparator: std::memcmp();
 - value count: 10k - 100k - 1M

Before each test we prepare vector of tuples storing truly random
values.

Here's the list of results obtained on my PC (i7-8700 12 X 4600 MHz):

Insertions: ~20-12M per second;
Find (no misses): ~58-16M* per second (find by key gives the same result);
Find (many misses): ~84-30M per second;
Iteration with dereference: ~450M per second;
Insertions after erase: ~50-17M* per second;
Find after erase: ~52-17M* per second (the same as without erase);
Delete: ~32-8M* per second.

* The first value is for 10k values in hash table; second - is for 1M.

Just to have some baseline here results for quite similar benchmark for
std::unordered_map (it is also included in source file):

Insertions: ~26-8M per second;
Find (no misses): ~44-11M per second;
Iteration with dereference: ~265-56M per second;
Find after erase: ~37-13M per second.

Part of #7338

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