Convert ProductionTypes from HashSet<string> to BitSet<ProductionType>

[pchote]

This is another major gameplay system that is still stuck using HashSet.

[chrisforbes]

A bunch of the things we would want to convert are currently restricted to a single value. Deferring a string -> bit value lookup to random runtime points is horrible -- it has to take a lock to ensure the bitset mapping isn't changing underneath it.

Do we want another type SingleBit<T> to go with BitSet<T> which can do this lookup at rules load time?