Generated classes take up more space in jar than needed

In glancing at one of our jars of generated Thrift class files, I noticed that it was pretty huge. Further digging showed that while certainly most of the bulk was attributable to the number of class files we have, part of it was due to lots of unnecessary internal classes. This is because we use a specific syntax for defining some maps inline in structs and enums.

Since this is only syntactic sugar, I think it would make sense to write a tiny bit of helper code and avoid the need for a dynamic internal class.