vm: put child invocation tree nodes into call element

As neo-project/neo-modules#657 currently does.

pkg/vm/invocation_tree.go

@@ -8,5 +8,5 @@ import (
 // you can see how contracts called each other.
 type InvocationTree struct {
 	Current util.Uint160      `json:"hash"`
-	Calls   []*InvocationTree `json:"calls,omitempty"`
+	Calls   []*InvocationTree `json:"call,omitempty"`
 }