message: Use const fn where applicable
diff: @@ -83,7 +83,7 @@ impl Subscription {
  /// # Ok(())
  /// # }
  /// ```
  - pub fn messages(&self) -> Iter<'_> {
  + pub const fn messages(&self) -> Iter<'_> {
  Iter { subscription: self }
  }
  @@ -98,7 +98,7 @@ impl Subscription {
  /// # Ok(())
  /// # }
  /// ```
  - pub fn iter(&self) -> Iter<'_> {
  + pub const fn iter(&self) -> Iter<'_> {
  Iter { subscription: self }
  }
  @@ -113,7 +113,7 @@ impl Subscription {
  /// # Ok(())
  /// # }
  /// ```
  - pub fn try_iter(&self) -> TryIter<'_> {
  + pub const fn try_iter(&self) -> TryIter<'_> {
  TryIter { subscription: self }
  }
  @@ -128,7 +128,7 @@ impl Subscription {
  /// # Ok(())
  /// # }
  /// ```
  - pub fn timeout_iter(&self, timeout: Duration) -> TimeoutIter<'_> {
  + pub const fn timeout_iter(&self, timeout: Duration) -> TimeoutIter<'_> {
  TimeoutIter {
  subscription: self,
  to: timeout,
  