compiler/iterator: add missing iterator.Concat function

We have a syscall for it, so it should be exposed.

pkg/compiler/syscall.go

@@ -108,6 +108,7 @@ var syscalls = map[string]map[string]string{
 		"GetExecutingScriptHash": "System.ExecutionEngine.GetExecutingScriptHash",
 	},
 	"iterator": {
+		"Concat": "Neo.Iterator.Concat",
 		"Create": "Neo.Iterator.Create",
 		"Key":    "Neo.Iterator.Key",
 		"Keys":   "Neo.Iterator.Keys",

pkg/interop/iterator/iterator.go

@@ -18,6 +18,17 @@ func Create(items []interface{}) Iterator {
 	return Iterator{}
 }
 
+// Concat concatenates two given iterators returning one that will range on
+// a first and then continue with b. Iterator positions are not reset for a
+// and b, so if any of them was already advanced by Next the resulting
+// Iterator will point at this new position and never go back to previous
+// key-value pairs. Concatenated iterators also remain completely independent
+// in results they return, so if both contain the same key you'll receive this
+// key twice when iterating. This function uses `Neo.Iterator.Concat` syscall.
+func Concat(a, b Iterator) Iterator {
+	return Iterator{}
+}
+
 // Key returns iterator's key at current position. It's only valid to call after
 // successful Next call. This function uses `Neo.Iterator.Key` syscall.
 func Key(it Iterator) interface{} {