Allow extending ToP4, clean its constructors

[vlstill]

• Replaced the cstring argument in ToP4 with std::filesystem::path. This is the breaking change.
• Cleaned up its constructors to avoid duplication.
• Allow re-using the dumper in ParserOptions with a descendant of ToP4 -- the way I did this is not exactly nice, but I don't see other clear way to create the instance on demand.
• The ToP4 pass is a good candidate for larger refactoring, but I don't have time for that now.

[asl]

but I don't see other clear way to create the instance on demand.

We'd probably go with some value-based semantics here. But it seems pretty heavyweight for a task. And essentially the same wrapped smart pointer. :)