Update encoder (#504) * pull `BROTLI_MAX_BACKWARD_LIMIT` to constants * split generic and Zopfli backward references code * pull hashers init and stitch invocation to encoder * make `dictionary_hash` a compilation unit * add `size hint` parameter * add new hasher * use `size hint` to pick new hasher for q4 * modernize clz guard (fix #495) * move `hash to binary tree` to separate file * add `Initialize` and `Cleanup` to all hashers * do not raise OOM if malloc(0) == NULL (fix #500)