Vexu
Add protection from calling `typeStr` with an invalid type
This checks before calling typeStr with the places that I could generate a crash, and adds a check for non-debug modes so that they'll return an obvious error instead of illegal behavior.
There are no tests because you need lots of errors in the first place to generate these crashes and I don't think the messages themselves would be very useful. I tested manually by making a builtin with a return type of .invalid and doing various operations on the returned value.
Sorry to make you check all potential places where this could happen but seeing the change now I agree that it'd be better to just check the type in typeStr and return an error.
