Using size feedback in the Allocator interface for flat_hash_map

Consider implementing [P0401R3 Providing size feedback in the Allocator interface](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0401r3.html) for `flat_hash_map` and friends. I've been using `flat_hash_map` with huge pages and P0401R3 can prevent wasting memory. I added support to my own library SPSCQueue and it can be done unobtrusively in C++17: https://github.com/rigtorp/SPSCQueue/blob/master/include/rigtorp/SPSCQueue.h#L37