Move some of the new logic out of `GPBUnknownField` so it will end up as a much
simpler "container", with all the serialization logic inside `GPBUnknownFields`
instead.
Also move some of the internal logic needed into static C functions to reduce the
ObjC class overhead of `GPBUnknownFields`.
This was all inspired by realizing during serialization related apis the `type` of
each field was being examined multiple times and reduces that in addition to reducing
the number of methods being invoked.
PiperOrigin-RevId: 650631975
`GPBUnknownFields` will be the eventually replacement for `GPBUnknownFieldSet`. This
introduces the type and the changes to `GPBUnknownField`.
The new api will preserve the wire ordering of unknown fields. This is now checked
in conformance tests.
While this adds the type changes and tests them, it does not yet wire the changes
in to the rest of the Runtime, so the conformance tests still done pass.
`GPBUnknownFieldSet` also hasn't been deprecated yet, that will come in later with
the wiring in to the runtime.
PiperOrigin-RevId: 648361455
Thomas Van Lenten