feat(sol-macro): add return value names to simple getters

[DaniPopes]

Example:

sol! {
    contract Counter {
        uint256 public number;
    }
}

// Before:
Counter::numberReturn { _0: U256 }
// After:
Counter::numberReturn { number: U256 }

See the added test

[DaniPopes]

This mismatches Solc-emitted ABI because it does not populate return names