Make RequestId, SlotId, Nonce, PurcahseId distinct types.
Add/modify conversions to support the distinct type (ABI encoding/decoding, JSON encoding, REST decoding).
Update tests
- rename `ContractId` to `SlotId`
- add `RequestId`, `PurchaseId`, `Nonce` types as aliases of `array[32, byte]`
- rename `Proving.contracts` to `Proving.slots`
- change signatures of `isSlotCancelled` and `isCancelled` to use `SlotId` and `RequestId` types, respectively.
- change all references to `RequestId`, `SlotId`, and `PurchaseId`
Eric Mastro
Mark Spanbroek
Michael Bradley