Make 'Callable' message of descriptors.proto extensible, extend it in
java_descriptors.proto with a JVM signature of the member. This is needed in
order for annotation deserializer to find out which member in the compiled
bytecode corresponds to which descriptor in the hierarchy.
Create a new module 'serialization.java' containing everything related to
Java-specific serialization of descriptors.
Add an extension point to DescriptorSerializer, allowing to perform
platform-specific serialization on a callable
Alexander Udalov