Macro fadroma::impl_contract_harness

macro_rules! impl_contract_harness {
    ($visibility:vis $name:ident, $module:ident) => { ... };
    ($visibility:vis $name:ident, $module:ident, $impl_struct:path) => { ... };
}

Generate a struct and implement ContractHarness for the given contract module. Alternatively, if using the derive contract macro, specify the third argument which is the implementation struct to use.