message: add missing APIs for Account's spending counter
diff: @@ -122,6 +122,9 @@ impl State {
  pub struct SpendingCounter(u32);
  impl SpendingCounter {
  + pub fn zero() -> Self {
  + SpendingCounter(0)
  + }
  fn increment(&self) -> Option<Self> {
  self.0.checked_add(1).map(SpendingCounter)
  }
  @@ -130,6 +133,11 @@ impl SpendingCounter {
  self.0.to_le_bytes()
  }
  }
  +impl From<u32> for SpendingCounter {
  + fn from(v: u32) -> Self {
  + SpendingCounter(v)
  + }
  +}
  /// Account Spending witness, which contains a
  /// cryptographic signature and a counter.
  