This significantly simplifies the internals of PrimitiveMut,
and removes the need to refactor BytesMut and ProtoStrMut
to have the same runtime branching.
PiperOrigin-RevId: 589292565
accessors_test.rs is getting a bit unwieldy, approaching 1K LOC. This CL breaks out the repeated tests into their own file (accessors_repeated_test.rs), as we expect the tests here to grow. This follows the precedent of `accessors_map_test.rs`.
PiperOrigin-RevId: 588554476