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/datetime/now_fact.go

@@ -102,12 +102,22 @@ func (obj *DateTimeFact) Stream(ctx context.Context) error {
 			return nil
 		}
 
+		result, err := obj.Call(ctx)
+		if err != nil {
+			return err
+		}
+
 		select {
-		case obj.init.Output <- &types.IntValue{ // seconds since 1970...
-			V: time.Now().Unix(), // .UTC() not necessary
-		}:
+		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 *DateTimeFact) Call(ctx context.Context) (types.Value, error) {
+	return &types.IntValue{ // seconds since 1970...
+		V: time.Now().Unix(), // .UTC() not necessary
+	}, nil
+}