Added Map.updateValue, closes #168
Added MultiMap[K, V].pop(K), closes #124
Added Map[K, V].updateValues(V => Option[W]): Map[K, W]
Added Map[K, V].keyExists(Predicate[K]): Boolean
Added Map[K, V].composeM(Map[C, K]): Map[C, V]
Added Map[K, V].updateKeys(K => Option[C]): Map[C, V]