vm: add MarshalJSON to the Stack

To easily dump it in a known format.

pkg/vm/stack.go

@@ -1,6 +1,7 @@
 package vm
 
 import (
+	"encoding/json"
 	"fmt"
 	"math/big"
 
@@ -334,3 +335,8 @@ func (s *Stack) popSigElements() ([][]byte, error) {
 	}
 	return elems, nil
 }
+
+// MarshalJSON implements JSON marshalling interface.
+func (s *Stack) MarshalJSON() ([]byte, error) {
+	return json.Marshal(stackToArray(s))
+}