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/serialization.java/serialization.java.iml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+    <orderEntry type="library" name="protobuf-java" level="project" />
+    <orderEntry type="module" module-name="serialization" />
+    <orderEntry type="library" name="annotations" level="project" />
+  </component>
+</module>
+