rpc: drop duplicating Invoke* structures

And use smartcontract.Parameter instead of vm.StackItem where
appropriate. Closes #689.

pkg/rpc/response/result/invoke.go

@@ -1,7 +1,7 @@
 package result
 
 import (
-	"github.com/CityOfZion/neo-go/pkg/vm"
+	"github.com/CityOfZion/neo-go/pkg/smartcontract"
 )
 
 // Invoke represents code invocation result and is used by several RPC calls
@@ -10,5 +10,5 @@ type Invoke struct {
 	State       string `json:"state"`
 	GasConsumed string `json:"gas_consumed"`
 	Script      string `json:"script"`
-	Stack       *vm.Stack
+	Stack       []smartcontract.Parameter
 }