message: add method to estimate the overhead to add some bytes in a given transaction
diff: @@ -107,11 +107,20 @@ impl LinearFee {
  /// Calculation of fees for a specific chosen algorithm
  pub trait FeeAlgorithm {
  + fn estimate_overhead(&self, num_bytes: usize) -> Result<Option<Fee>>;
  +
  fn calculate_for_txaux(&self, txaux: &TxAux) -> Result<Fee>;
  fn calculate_for_txaux_component(&self, tx: &Tx, witnesses: &Vec<TxInWitness>) -> Result<Fee>;
  }
  impl FeeAlgorithm for LinearFee {
  + fn estimate_overhead(&self, num_bytes: usize) -> Result<Option<Fee>> {
  + let msz = Milli::integral(num_bytes as u64);
  + let fee = self.coefficient * msz;
  + let coin = Coin::new(fee.to_integral())?;
  + Ok(Some(Fee(coin)))
  + }
  +
  fn calculate_for_txaux(&self, txaux: &TxAux) -> Result<Fee> {
  // the only reason the cbor serialisation would fail is if there was
  // no more memory free to allocate.
  