[Enhancement] Add support for MapSets

[kerryb]

It looks like Hammox doesn’t currently support typespecs that use MapSet.t, because these use Elixir’s :sets module internally, and that declares its types as records.

I’ve created a trivial project to demonstrate the issue – I’m not sure I’ve got the skills to figure out how to fix it, but I might give it a try if I get time.