Switch world age fields to ulong

[Keno]

They are declared as size_t in the header, which is unsigned.

[vtjnash]

Also base/reflection.jl accessors, and probably need to fix Serializer to be compatible. But otherwise, lgtm if it passes on CI.

[Keno]

and probably need to fix Serializer to be compatible

Are you thinking we need to write custom serializers/deserializers for every type we're changing here? I'm not really sure how feasible that is. CodeInstance wasn't even a thing in earlier 1.0 instances. Also world ages should never really have been serialized in the first place, since they won't be meaningful after a reload.

[Keno]

So @JeffBezanson @vtjnash any guidance on what needs to change in the Serializer?