Add DontSerialize Trait/Annotation

This adds a DontSerialize Annotation mixin that can be mixed into
annotations that a user does not want to be included in JSON
serialization. This differs subtly from the Unserializable mixin in
that unserializable indicates that an annotation cannot be serialized
while don't serialize indicates that an annotation has some
attribute that makes it a bad idea to serialize, e.g., it's very large
and not useful like FirrtlCircuitAnnotation.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>