Allow instance definitions with variables constrained on local classes

[YarinHeffes]

Currently, the following block does not compile,

(coalton-toplevel
  (define-class (C :T)
    (m (:T -> :T -> Boolean)))
  (define-instance (C :T => Eq :T)
    (define (== x y) (m x y)))

because the instance definition must either be an instance of a class from the current package or refer to a type from the current package. However, it is not currently sufficient for the instance to refer to type variables constrained by classes from the current package.

error: Invalid orphan instance
...
instances must be defined in the same package as their class or reference one or more types in their defining package