Add generic `ContractInterface` implementation for contracts built from only `ComposableState`

[nemonical]

Currently, to write a contract using freenet-scaffold, you need to both write the ComposableStates and the ContractInterface impl. If the contract has only a single root composableState, this impl will be the same every time. I think it would be a good idea to provide a standard Contract<T: CompostbleState> to remove the boilerplate.

[nemonical]

I have created a fork to try my hand at implementing my proposal, it's at https://github.com/nemonical/freenet-scaffold/tree/nemonical-generic-contract-interface-impl.