Make proptests generic over `BitSetValueTree`

[saik0]

Then we get the same test suite for free on

• RoaringTreemap
• Store
• ArrayStore
• BitmapStore
• RunStore (future) Add support for run containers #12

AND we'd be able to add new generator/shink strategies, for example generating two Stores such that lhs is a RunStore and rhs is an ArrayStore, we could trivially test set ops for every combination!

[saik0]

@Kerollmops @josephglanville

Would this help your work in #66?