[C++] Refactor uint128

[Yannic]

This makes it compatible with absl::uint128

[acozzette]

Thanks, @Yannic.

[lipnitsk]

This broke external code depending on descriptor.pb.h.

Step #4: In file included from /src/protobuf-install/include/google/protobuf/descriptor.pb.h:23:
Step #4: In file included from /src/protobuf-install/include/google/protobuf/io/coded_stream.h:148:
Step #4: /src/protobuf-install/include/google/protobuf/stubs/logging.h:34:10: fatal error: 'google/protobuf/stubs/int128.h' file not found
Step #4: #include <google/protobuf/stubs/int128.h>
Step #4:          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32763
Link: https://oss-fuzz-build-logs.storage.googleapis.com/log-b9d0c99a-00b5-4af7-862c-f16a5a11c457.txt

[acozzette]

@lipnitsk Thanks for the heads-up, let's roll back this change until we have a chance to come up with a fix. I will sent out a pull request in just a second.