1. use `ProviderError` to catch contract errors instaed of `JsonRpcProviderError`.
2. Add tests for `isProofRequired`, `willProofBeRequired`, and `getProofEnd` when slot is empty (and is call is reverted)
3. bump nim-ethers to branch that has new `reverts` API and `ProviderError` support.
4. Update to new `reverts` API.
- 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`