Improve/Revamp Macro Support

[JesseCoretta]

Describe the solution you'd like
Macros support is sloppy and implemented almost as an afterthought. It should be revamped in a manner similar to the Extension redesign, which discontinued use of map[string][]string types in favor of proper collection types.

Describe alternatives you've considered
Simply not using Macros will work most of the time, however certain RFCs, such as RFC2307, do require basic Alias-To-OID resolution for certain definitions during the Marshal (parsing) process. Therefore, outright abandoning macro support is not feasible ... its here to stay.

Leaving the current solution as it is would be fine too, but a revamp certainly couldn't hurt.

[JesseCoretta]

Implemented by #16