Require a colon before 'group' or 'union' except for unnamed unions as well as the legacy case of a named union with an ordinal (where the colon is optional).

Make enum syntax match docs, make enum lists work correctly, add enum tests, enforce maximum ordinals on enum values and interface methods, change the maximum field ordinal to 16-bit to match docs (now all ordinal limits match).

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...