HyperLogLog has an updateInto method which updates a mutable buffer representing its current state, instead of doing immutable operations. This allows for an efficient implementation of sumOption. It would be helpful to standardize this so that we could use it from eg SummingCache. see #290

https://github.com/twitter/algebird/blob/develop/algebird-core/src/main/scala/com/twitter/algebird/HyperLogLog.scala#L241