SmartContractModule: ContractsToBillAt property should be a map towards sets of contract ids

[brandonpille]

The property ContractsToBillAt is a map from blocknumber towards a vector of contracts to bill. As a contract should only be billed once per block it makes more sense to change ContractsToBillAt to map towards a set of contracts to bill.

To be defined: what collection to use, either using vectors as sets (https://substrate.recipes/vec-set.html) or https://docs.rs/sp-std/latest/sp_std/collections/btree_set/struct.BTreeSet.html or something else.