add serialized-size function to core

Adds a `serialized-size` function to the core namespace, as well as the associated getter in PersistentProtocolBufferMap.  Simply calls through to [MessageLite#getSerializedSize()](https://code.google.com/p/protobuf/source/browse/tags/2.4.1/java/src/main/java/com/google/protobuf/MessageLite.java#76).