Use VAST's fallthrough visitor

[PappasBrent]

Macroni's visitors should inherit from VAST's new fallthrough_visitor struct so that they only need to define the visit methods relevant to them.

Currently, each of Macroni's visitors that inherit from visitor_base need to define all their visit method in order to be concrete structs. However, this adds a lot of boilerplate code since Macroni's visitors only do special actions for one or two visit methods, and just return nullptr for all else. So to reduce boilerplate Macroni's visitors should inherit from a struct that already defines all its visitors to return nullptr. There are PRs open which use a custom-defined empty visitor; however once VAST merges this commit Macroni's structs should just inherit from the fallthrough_visitor.