HyperLogLogPlusPlus should be serializable

To allow inclusion of this in systems like Apache Beam and Apache Flink the HyperLogLogPlusPlus class should be serializable.
I ran into this while trying to implement a UDF for Apache Flink, there serializability is mandatory.

Also when trying to put an instance of the HyperLogLogPlusPlus class through a simple Beam pipeline resulted in
```
Caused by: java.io.NotSerializableException: com.google.zetasketch.HyperLogLogPlusPlus
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)

```