* Optimize appending another Parts container
* Remove redundant/verbose comments
* Change r-value args to move-only types into l-value args for
readability
* Deprecate `AtRef(int)`, redundant, just dereference at call site
* Deprecate `AddPart(Message*)`, avoid owning raw pointer args
* Add various const overloads
* Add `Empty()` and `Clear()` member functions
* Add `noexcept` where applicable
