vm: add invocation tree tracing

pkg/vm/invocation_tree.go

@@ -0,0 +1,12 @@
+package vm
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+// InvocationTree represents a tree with script hashes, traversing it
+// you can see how contracts called each other.
+type InvocationTree struct {
+	Current util.Uint160
+	Calls   []*InvocationTree
+}