Support deserializing into existing histogram

[jonhoo]

The current deserialization example is a little sad, in that it needs to allocate a new histogram for each of the intermediate deserialized histograms. It would be awesome if there was also a way to deserialize directly into an existing histogram (essentially deserialize + add without an intermediate collection). I don't know if this is doable given the serialization format though..

[marshallpierce]

I think it's probably doable.