Improving function call return types

With the inclusion of core#855 return values of function calls with a singular value is more intuitive and easier to work with.

Consider the following example of reading the balance of an ERC20:

sol! {
    #[sol(rpc)]
    contract ERC20 {
        // Note: Only a single value is being returned
        function balanceOf(address) returns (uint256);
    }
}

Before

Calling the balanceOf fn would return a struct balanceOfReturn which encapsulated the actual balance value.

// .. snip ..
let balance_return: balanceOfReturn = erc20.balanceOf(owner).await?;
 
let actual_balance = balance_return._0;

After

Calling the balanceOf fn would now yield the balance directly instead of a struct wrapping it.

// .. snip ..
let balance: U256 = erc20.balanceOf(owner).await?;

It is important to note that this change only applies to function calls that have a singular return value.

Function calls that return multiple values have their return types unchanged, i.e they still return a struct with values inside it.

sol! {
    function multiValues() returns (uint256 a, address b, bytes c);
}
 
// The above function call will have the following return type.
 
pub struct multiValuesReturn {
    pub a: U256,
    pub b: Address,
    pub c: Bytes,
}