Support for typed return values and output with custom types

[digorithm]

This PR introduces decoded and type-safe output for contract calls.

It also adds support for structs as return values for contract calls.

Before this, contract calls through the SDK would just return a u64.

Now, we're fully leveraging the Tokenizable and Detokenize traits
that were designed earlier to decode the return values coming from
contract calls. For instance:

let response: bool = contract_instance.is_even(10).call().await.unwrap();

let my_struct = MyStruct { foo: 10, bar: true };
let response: MyStruct = contract_instance
                            .return_my_struct(my_struct)
                            .call()
                            .await
                            .unwrap();