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

Rename the 'Message' interfaces to Arena and make them internal.  Make a new, intuitive 'Message' interface for creating / reading messages.

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