Both intrinsics and primitives should be allowed to be defined in-source

To do this, there should be an attribute `#[intrinsic]` to label something as being an intrinsic, and it should be "linked" appropriately on the compiler side so that the intrinsic can be accessed symbolically.

For example, you should be able to do `Term::Call(Intrinsic::TypeOf.fn(), [ty])`

This should also allow defining a lot of the primitives in-source too.