Header-only library that provides 0 cost initialization for immutable
containers, fixed-size containers, and various algorithms.

Frozen provides:

  * immutable (a.k.a. frozen), constexpr-compatible versions of
    std::set, std::unordered_set, std::map and std::unordered_map.

  * fixed-capacity, constinit-compatible versions of std::map and
    std::unordered_map with immutable, compile-time selected keys
    mapped to mutable values.

  * 0-cost initialization version of std::search for frozen needles
    using Boyer-Moore or Knuth-Morris-Pratt algorithms.

The unordered_* containers are guaranteed perfect (a.k.a. no hash
collision) and the extra storage is linear with respect to the
number of keys.

Once initialized, the container keys cannot be updated, and in
exchange, lookups are faster. And initialization is free when
constexpr or constinit is used :-).