Add return struct support from abigen function signatures

[alxiong]

Is your feature request related to a problem? Please describe.
Currently for a function in contract that returns a struct, its abigen! generated signature would have a tuple as its return types in rust.

pragma solidity >=0.8.0;

contract Test {
  struct Person {
    string name;
    uint age;
  }
  function defaultPerson() public pure returns (Person memory) {
    return Person("Alice", 20);
  }
}

contract's ABI

[
	{
		"inputs": [],
		"name": "defaultPerson",
		"outputs": [
			{
				"components": [
					{
						"internalType": "string",
						"name": "name",
						"type": "string"
					},
					{
						"internalType": "uint256",
						"name": "age",
						"type": "uint256"
					}
				],
				"internalType": "struct Hello.Person",
				"name": "",
				"type": "tuple"
			}
		],
		"stateMutability": "pure",
		"type": "function"
	}
]

and in my Rust code:

// assume `contract` has been instantiated 
let person = contract.defaultPerson().call().await?;
    // ^^^ this is a tuple of (String, U256), instead of struct Person from abigen!

Describe the solution you'd like
Ideally we could also have return types matched the solidity function signature instead of ABIEncoderV2's representation.

Thanks a lot! @mattsse