Actually fix the bug, which was a doozy:  OrphanBuilder::tag was sometimes initialized using WirePointer::setKindAndTarget(), but since the tag didn't live inside the target segment, this used illegal pointer arithmetic.  The target is never read from an orphan tag anyway, so I thought it would be no big deal.  But it turns out Clang actually optimizes under the assumption that pointer arithmetic returns a whole value.  As a result, on 32-bit system where 64-bit values are only 32-bit aligned, the tag and target might not have been a whole number of words apart, and the extra bit actually found its way into the 'kind' bits, causing e.g. a struct pointer to become an invalid far pointer.  Crash.  The fix required refactoring to ensure that setKindAndOffset() is never used for orphan tags.

Fix obscure bug where allocating an empty struct causes trouble.  For reasons I don't yet fully understand, this actually caused a crash on 32-bit OSX builds (but not 64-bit, nor 32-bit Linux).

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.