This change adds delete, clear, serialize, parse, copy_from, and merge_from
operations to the runtime. Since these operations can all be implemented easily
on the `MessageLite` interface, we can use a common implementation in the
runtime instead of generating per-message thunks for all of these.
I suspect this will also make it possible to remove some of our generated trait
implementations and replace them with blanket implementations, but I will leave
that for a future change.
PiperOrigin-RevId: 665910927
Adam Cozzette