boost static_map

This document proposes an addition to Boost C++ Libraries -- a compile-time hash table. There are multiple good implementations of unordered associate containers (e.g. std::unordered_map, Google's sparsehash). These implementations provide both lookup and insertion/deletion functionality. They are, however, not the perfect fit for the case when the contents of the container are fixed upon construction or even known at compile time. std::vector vs. std::array is a good analogy here. We propose a static_map -- an associate container with focus on constexpr usage.