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).

New method isFoo() exists for union fields only and returns true if foo is the active union field.

Switch to the new compiler.  All tests pass\!  Need to submit bootstrap copies of the generated code used by the compiler itself.  These bootstrap copies are still built by the old compiler at the moment.