Invoke helper

[tj]

something similar to the following snippet, reduce boilerplate for the simple case.

// Invoke function.
func Invoke(client *lambda.Lambda, name string, in interface{}, out interface{}) error {
    b, err := json.Marshal(in)
    if err != nil {
        return err
    }

    res, err := client.Invoke(&lambda.InvokeInput{
        FunctionName: &name,
        Qualifier:    aws.String("current"),
        Payload:      b,
    })

    if err != nil {
        return err
    }

    return json.Unmarshal(res.Payload, out)
}