Enforce ToArrow trait on relevant indexes

[Max-Meldrum]

Related to #210.

One option is to force ToArrow trait on ArconType. But this would mean limiting what data can be sent through the dataflow. Instead the better option I think is to enforce ToArrow on state indexes as this is where it will be used.

pub trait IndexValue: Value + ToArrow {}
impl<T> IndexValue for T where T: Value + ToArrow {}