message: [pdsl_core] Add storage::Vec::get_mut(&mut self) -> Option<&mut T>
diff: @@ -188,6 +188,15 @@ where
  .and_then(|n| self.cells.get(n))
  }
  + /// Returns a mutable reference to the `n`-th element of the vector.
  + ///
  + /// Returns `None` if `n` is out of bounds.
  + pub fn get_mut(&mut self, n: u32) -> Option<&mut T> {
  + self
  + .within_bounds(n)
  + .and_then(move |n| self.cells.get_mut(n))
  + }
  +
  /// Mutates the `n`-th element of the vector.
  ///
  /// Returns a reference to the mutated element.
  