lang: Plumb through a context into unification

If we have a long type unification, we might want to cancel it early.
This also helps us visualize where we want context to be seen.

lang/lang.go

@@ -232,7 +232,7 @@ func (obj *Lang) Init() error {
 		Debug:  obj.Debug,
 		Logf:   logf,
 	}
-	unifyErr := unifier.Unify()
+	unifyErr := unifier.Unify(context.TODO())
 	obj.Logf("type unification took: %s", time.Since(timing))
 	if unifyErr != nil {
 		return errwrap.Wrapf(unifyErr, "could not unify types")