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)