IF: Use enum to track which action receipt digest to calculate

[heifner]

Suggested alternative for tacking which digest to caculate. Even better I think would be to create:

struct action_receipt_digests {
   enum class store_action_digests { legacy, savanna, both };
   // add() etc
   store_action_digests          action_digests_to_store;
   std::optional<digests_t>      executed_action_receipt_digests_l;
   std::optional<digests_t>      executed_action_receipt_digests_s;
};

[heifner]

struct was added.

[ericpassmore]

Note:start group: IF category: INTERNALS summary: Improve digest tracking using enums. Note:end