[NFC] Refactor out subtyping discovery code

[kripken]

This implements an idea I mentioned in the past, to extract the subtyping discovery
code out of Unsubtyping so it could be reused elsewhere. Example possible uses:
the validator could use to remove a lot of code, and also a future PR of mine will
need it. Separately from those, I think this is a nice refactoring as it makes Unsubtyping
much smaller.

This just moves the code out and adds some C++ template elbow grease as needed.

[tlively]

This LGTM % some small comments. It would be very exciting if this let us simplify the validator.

[tlively]

Looks like the formatting could be more consistent here.

[tlively]

Perhaps I'll find out below, but what is the benefit of this over just extracting the type of the cast expression and using noteCast(Expression, Type) in this case?

[kripken]

This PR tries to keep the info as "high-level" as possible, that is, not to lose information. We might want the extra information here to say something about the expression that fails to validate, if we use this in the validator some day (though, if we have the cast anyhow, we can do cast->value).

[tlively]

Do we have any other cases where we take a parent as a parameter instead of a subclass as a parameter?

[kripken]

Oh, maybe "Parent" is a confusing name here, yeah. It's the same as the usual pattern.

[tlively]

I think that if we wanted to use this in the validator, we would want to note the "subtyping" for non-ref values as well.

[kripken]

Good point, yeah - maybe when we get there, this could be renamed to ReferenceSubtypingDiscoverer, and we'd have a subclass that is a general one that adds non-refs?