Open source hash_map with unparalleled performance/memory usage ratio

It's an improved version of Google's sparsehash, with better performance and actual key deletion (instead of erasure), though with a slight memory hit.

It links to a performance comparison of their sparse++ with g++ version 4.8.4's std::unordered_map, Boost version 1.55's unordered_map, and Google's Sparsehash and cpp-btree libraries, at https://github.com/greg7mdp/sparsepp/blob/master/bench.md .

(While a link to the benchmark is in the README, I managed to overlook the link because it doesn't stand out.)