Fix bug where only the first import made it into the import table.  Can't believe this wasn't tested.  Sigh.

Improve eval benchmark message structure to waste fewer objects (for both protobuf and capnproto, but this turns out to help capnproto more than protobufs).

Basic code generation starting to work.  My machine just did something weird so I'm submitting and pushing in case of imminant hardware failure...