[#907] morph/event: Implement Op to string converter

Implement `StringFromOpcode` function that tries to retrieve `string` to
`Op`. Add a comment about neo-go source code that is used for implementation
of converters.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/morph/event/opcodes.go

@@ -25,6 +25,8 @@ func (o Op) Param() []byte {
 	return o.param
 }
 
+// Below are the functions which reverse the results of github.com/nspcc-dev/neo-go/pkg/vm/emit.Array function.
+
 // BytesFromOpcode tries to retrieve bytes from Op.
 func BytesFromOpcode(op Op) ([]byte, error) {
 	switch code := op.Code(); code {
@@ -48,3 +50,10 @@ func IntFromOpcode(op Op) (int64, error) {
 		return 0, fmt.Errorf("unexpected INT opcode %s", code)
 	}
 }
+
+// StringFromOpcode tries to retrieve string from Op.
+func StringFromOpcode(op Op) (string, error) {
+	// strings are emitted like bytes
+	data, err := BytesFromOpcode(op)
+	return string(data), err
+}