Fix define-instance context bug (1340)

[YarinHeffes]

Fixing the following bug:

(coalton-toplevel
  (define-class (C :col :elem))
  (define-class (Eq :elem => EqC :col :elem))
  (define-instance (C (List :elem) :elem))
  (define-instance (Eq :elem => EqC (List :elem) :elem)))

This block previously would not compile, because the typechecker checked that :elem satisfies Eq :elem before checking the predicates provided in the define-instance. See #1340.

[Jason94]

Fixed the issue I was having working on collections, so looks good to me 👍