Group all structs derived from a module into a single trait.

[zie1ony]

trait Gens {
    type HostRef: Deployer;
    type ContractRef: ContractRef;
    type InitArgs: InitArgs;
}

impl Gens for Controller {
    type HostRef = ControllerHostRef;
    type ContractRef = ControllerContractRef;
    type InitArgs = ControllerInitArgs;
}