Make rulebase efficiently serializable

RuleBase in <2.1 was not efficiently serialized, such that the rule code was compiled each time. With 2.5 and new DDJ feature, rulebase is not serializable without the a proxy to store and retrieve.

Add a new SerializableRuleBaseProxy to allow RuleBase to be serializable to and from a collection of DDJ compiled rulesets. So compilation only occurs once. A future enhancement may be to serialize the RETE with the ruleset itself in the DDJ, for even faster loading.