Add option for eliding property metadata from messages

Adds an option to protoc `--objc_opt=elide_message_metadata` to remove all the property
metadata from message classes. This significantly reduces the codegen size of the clases.
The downside is that iterating through properties using objective c runtime calls will no
longer function. This is mitigated by the fact that most (all?) of the information that
folks are interested in can be extracted via the message descriptor.

We do this by defining our own classes using the `GPB_MESSAGE_SUBCLASS_IMPL` macro.