Switch to using int2int maps in XmiSerializationSharedData

The XmiSerializationSharedData structure is used to record information when deserializing so that subsequent serialization can be done with identical xmiIds, as well as recording which non-shared arrays/lists are encoded as embedded values.

Change the data structures used for this to use Int2IntHashMap, and change the way values are store to be ints, not Strings. This should improve space and time for Xmi and Json serialization.