Changed emitter to use named formatting parameters in one place I'm about to modify.
Adding a base class that internal user-defined structs can inherit from.  Should help with versioning structs.
Struct fields now only hold references.
Struct now inherits from AtomicReferenceVariable, and we added a
method to serialize_contents of InternalStructs.  Still need to
actually ensure that serialize_contents gets called on
internal_struct.