We have used i64 before because that is the size of an integer in
SQLite. However, I believe that for large enough databases, the chances
of collision here are just too high. The effect of a collision is the
database silently returning incorrect data in the materialized view.
So now that everything else is working, we should move to i128.
The Materialized View code had custom serialization written so we could
move this code forward. Now that we have many operators and the views
work, replace it with something saner.
The main insight is that if we transform the AggregateState into Values
before the serialization, we are able to just use standard SQLite
serialization for the values. We then just have to add sizes, codes for
the functions, etc (which are also represented as Values).
