Deserialize annotations on class members

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

compiler/frontend.java/frontend.java.iml

@@ -13,6 +13,7 @@
     <orderEntry type="module" module-name="runtime" />
     <orderEntry type="library" name="javax.inject" level="project" />
     <orderEntry type="module" module-name="serialization" />
+    <orderEntry type="module" module-name="serialization.java" />
   </component>
 </module>