RFC: Subtype Checking

[Theodus]

Add a language construct allowing programmers to get information on subtyping relationships.

https://github.com/ponylang/rfcs/blob/master/text/0026-subtype-checking.md

[Praetonus]

I'm going to implement the iftype expression first since the change is straightforward. Function specialisations will require more work and will come later.

[SeanTAllen]

@jemc do you know how much of this was implemented?

[jemc]

The iftype conditional was implemented: https://github.com/ponylang/rfcs/blob/main/text/0026-subtype-checking.md#the-iftype-conditional

Specialized generic functions were not implemented: https://github.com/ponylang/rfcs/blob/main/text/0026-subtype-checking.md#specialised-generic-functions

They ended up being more complex than Benoit originally anticipated, and also Benoit is no longer involved with this project.