lang: Improve graph shape with speculative execution

Most of the time, we don't need to have a dynamic call sub graph, since
the actual function call could be represented statically as it
originally was before lambda functions were implemented. Simplifying the
graph shape has important performance benefits in terms of both keep the
graph smaller (memory, etc) and in avoiding the need to run transactions
at runtime (speed) to reshape the graph.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/types/value.go

@@ -1321,6 +1321,9 @@ func (obj *FuncValue) Value() interface{} {
 // inappropriate input types, or if it returns an inappropriate output type.
 func (obj *FuncValue) Call(ctx context.Context, args []Value) (Value, error) {
 	// cmp input args type to obj.T
+	if obj.T == nil {
+		return nil, fmt.Errorf("the type is nil")
+	}
 	length := len(obj.T.Ord)
 	if length != len(args) {
 		return nil, fmt.Errorf("arg length of %d does not match expected of %d", len(args), length)