Refactor topdown ref bindings
Refactor topdown ref bindings

These changes refactor topdown to use separate data structure to store
ref bindings.

Eva-xiaohui-luo/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,tsandall/opa,Eva-xiaohui-luo/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,timothyhinrichs/opa,open-policy-agent/opa,open-policy-agent/opa,timothyhinrichs/opa,timothyhinrichs/opa,open-policy-agent/opa,timothyhinrichs/opa,Eva-xiaohui-luo/opa,timothyhinrichs/opa