call macro applies to impl methods for contract method call.
contract defines basic struct as a programming model of a contract.
Fields are data representation of contract storage.
The macro contract_field can generate impl so that nested struct can be supported in contract struct.
contract_methods defines impl for the contract struct.
Methods declared in the impl are callable by Transaction Command Call if their visibility is pub.
use_contract provides smart contract developers a way to make cross contract calls by using
public functions from other smart contracts in the form of traits.