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/sys/load_fact.go

@@ -103,23 +103,32 @@ func (obj *LoadFact) Stream(ctx context.Context) error {
 			return nil
 		}
 
-		x1, x5, x15, err := load()
+		result, err := obj.Call(ctx)
 		if err != nil {
-			return errwrap.Wrapf(err, "could not read load values")
-		}
-
-		st := types.NewStruct(types.NewType(loadSignature))
-		for k, v := range map[string]float64{"x1": x1, "x5": x5, "x15": x15} {
-			if err := st.Set(k, &types.FloatValue{V: v}); err != nil {
-				return errwrap.Wrapf(err, "struct could not set key: `%s`", k)
-			}
+			return err
 		}
 
 		select {
-		case obj.init.Output <- st:
-			// send
+		case obj.init.Output <- result:
 		case <-ctx.Done():
 			return nil
 		}
 	}
 }
+
+// Call this fact and return the value if it is possible to do so at this time.
+func (obj *LoadFact) Call(ctx context.Context) (types.Value, error) {
+	x1, x5, x15, err := load()
+	if err != nil {
+		return nil, errwrap.Wrapf(err, "could not read load values")
+	}
+
+	st := types.NewStruct(types.NewType(loadSignature))
+	for k, v := range map[string]float64{"x1": x1, "x5": x5, "x15": x15} {
+		if err := st.Set(k, &types.FloatValue{V: v}); err != nil {
+			return nil, errwrap.Wrapf(err, "struct could not set key: `%s`", k)
+		}
+	}
+
+	return st, nil
+}