Layout for fixed size vs. dynamic size fields

[jfallows]

The flyweight generator allows fixed and dynamic size fields to have any desired ordering, making the determination of field offsets complex.

If all fixed size fields (including fixed length octet arrays) were ordered before all dynamic size fields, the generator code would be simplified and the runtime performance would improve accordingly.

The ordering of fixed size fields relative to other fixed size fields, and the ordering of dynamic size fields relative to other dynamic size fields should follow the IDL.