Implement transaction signing

[digorithm]

Adds transaction signing to the wallet API:

 let wallet = LocalWallet::new_from_private_key(secret).unwrap();

let input_coin = Input::coin(
   // ...
);

let output_coin = Output::coin(
 // ..
);

let tx = Transaction::script(
   // ..
);

let signature = wallet.sign_transaction(&tx).await.unwrap();

Closes #69.

[iqdecay]

LGTM