message: allow creating a transaction builder from a transaction
diff: @@ -71,6 +71,12 @@ impl TransactionBuilder<Address, tx::NoExtra> {
  }
  }
  +impl<Address, Extra> From<tx::Transaction<Address, Extra>> for TransactionBuilder<Address, Extra> {
  + fn from(tx: tx::Transaction<Address, Extra>) -> Self {
  + TransactionBuilder { tx }
  + }
  +}
  +
  impl<Extra: Clone> TransactionBuilder<Address, Extra> {
  /// Create new transaction builder.
  