Inline structs/strong type alias

Tracking discussion in https://groups.google.com/forum/#!topic/capnproto/KFPRyvmc33c, would be nice if it were possible to define inlineable structs that behaved identically to a group but provided a strong name that could be reused multiple times.

Considerations:

• Do we want to support only "sealed" types or should we support a general mechanism to provide named groups?
• What is the complexity this adds to cap'n'proto?