lang: core, funcs: Port some functions to CallableFunc API

Some modern features of our function engine and language might require
this new API, so port what we can and figure out the rest later.

lang/core/test/oneinstance_fact.go

@@ -243,13 +243,24 @@ func (obj *OneInstanceFact) Init(init *facts.Init) error {
 func (obj *OneInstanceFact) Stream(ctx context.Context) error {
 	obj.init.Logf("Stream of `%s` @ %p", obj.Name, obj)
 	defer close(obj.init.Output) // always signal when we're done
+
+	result, err := obj.Call(ctx)
+	if err != nil {
+		return err
+	}
+
 	select {
-	case obj.init.Output <- &types.StrValue{
-		V: msg,
-	}:
+	case obj.init.Output <- result:
 	case <-ctx.Done():
 		return nil
 	}
 
 	return nil
 }
+
+// Call this fact and return the value if it is possible to do so at this time.
+func (obj *OneInstanceFact) Call(ctx context.Context) (types.Value, error) {
+	return &types.StrValue{
+		V: msg,
+	}, nil
+}