compiler/interop: add support for working with witnesses

pkg/compiler/syscall.go

@@ -69,6 +69,7 @@ var syscalls = map[string]map[string]string{
 		"GetReferences": "Neo.Transaction.GetReferences",
 		"GetScript":     "Neo.InvocationTransaction.GetScript",
 		"GetType":       "Neo.Transaction.GetType",
+		"GetWitnesses":  "Neo.Transaction.GetWitnesses",
 	},
 	"asset": {
 		"Create":       "Neo.Asset.Create",
@@ -114,4 +115,7 @@ var syscalls = map[string]map[string]string{
 		"Value":  "Neo.Iterator.Value",
 		"Values": "Neo.Iterator.Values",
 	},
+	"witness": {
+		"GetVerificationScript": "Neo.Witness.GetVerificationScript",
+	},
 }

pkg/interop/transaction/transaction.go

@@ -7,6 +7,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/interop/attribute"
 	"github.com/nspcc-dev/neo-go/pkg/interop/input"
 	"github.com/nspcc-dev/neo-go/pkg/interop/output"
+	"github.com/nspcc-dev/neo-go/pkg/interop/witness"
 )
 
 // Transaction represents a NEO transaction, it's an opaque data structure
@@ -72,3 +73,10 @@ func GetOutputs(t Transaction) []output.Output {
 func GetScript(t Transaction) []byte {
 	return nil
 }
+
+// GetWitnesses returns a slice of witnesses of a given Transaction. Refer to
+// witness package on how to use them. This function uses
+// `Neo.Transaction.GetWitnesses` syscall.
+func GetWitnesses(t Transaction) []witness.Witness {
+	return []witness.Witness{}
+}

pkg/interop/witness/witness.go

@@ -0,0 +1,14 @@
+/*
+Package witness provides functions dealing with transaction's witnesses.
+*/
+package witness
+
+// Witness is an opaque data structure that can only be created by
+// transaction.GetWitnesses and representing transaction's witness.
+type Witness struct{}
+
+// GetVerificationScript returns verification script stored in the given
+// witness. It uses `Neo.Witness.GetVerificationScript` syscall.
+func GetVerificationScript(w Witness) []byte {
+	return nil
+}