Delete default construcrtor on builders, so that people don't incorrectly try to just declare a Builder on the stack and then start setting its fields.  Instead, a Builder can be explicitly assigned to nullptr, which makes it clearer that it isn't safe to use.

Rename non-group again, now to 'slot', which is much nicer.  Also introduce fixed lists as a third kind of field (but not implemented yet).

Extend 'capnp const' to support evaluating field default values as well as inner members of a value.

Make 'capnp decode' heuristically detect and warn when the wrong format flag (e.g. --packed, --flat) is used.