Immutable Transaction data structure

[debris]

Instead of having a single Transaction structure, we could have Transaction and SignedTransaction. They could look like this:

pub struct Transaction {
  pub nonce: U256,
  // ...
}

impl Transaction {
  fn sign(self) -> SignedTransaction {
    // signing is here
  }
}

pub struct SignedTransaction {
  t: Transaction, // or Transaction fields
  v: u8,
  r: U256,
  s: U256,
  hash: H256,
  sender: Address
}

It would reduce number of places where we check if transaction is correctly signed, to 1.