compiler|transaction: fix transaction.GetScript build, add to interop

There is no such syscall as Neo.Transaction.GetScript and GetScript should be
available for contract's use.

pkg/compiler/syscall.go

@@ -72,7 +72,7 @@ var syscalls = map[string]map[string]string{
 		"GetOutputs":      "Neo.Transaction.GetOutputs",
 		"GetReferences":   "Neo.Transaction.GetReferences",
 		"GetUnspentCoins": "Neo.Transaction.GetUnspentCoins",
-		"GetScript":       "Neo.Transaction.GetScript",
+		"GetScript":       "Neo.InvocationTransaction.GetScript",
 	},
 	"asset": {
 		"Create":       "Neo.Asset.Create",

pkg/interop/transaction/transaction.go

@@ -48,3 +48,10 @@ func GetInputs(t Transaction) []input.Input {
 func GetOutputs(t Transaction) []output.Output {
 	return []output.Output{}
 }
+
+// GetScript returns the script stored in a given Invocation transaction.
+// Calling it for any other Transaction type would lead to failure. It uses
+// `Neo.InvocationTransaction.GetScript` syscall.
+func GetScript(t Transaction) []byte {
+	return nil
+}