Move exception classes for layout transitions into separate file

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Introduce ObjectWithoutFields

This avoids a circular dependency for imports with Object, and is a memory optimization for really small objects.

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Use the nilObject global, don't pass it around

Signed-off-by: Stefan Marr <git@stefan-marr.de>