Macro validate 

macro_rules! validate {
    ( $( $n:expr => $v:expr => [ $( $c:expr ),* ] ),* ) => { ... };
}

validate! macro validates given fields and its inputs.

Example

    let name = "lorem ipsum".to_string();
    let description = Some("lorem ipsum dolor sit amet".to_string());

    let result = validate! {
        "name" => name => [length::max(3)]
    };
    assert!(result.is_err());

    let Error(out) = result.unwrap_err();
    assert_eq!(vec![
        Feedback {
            field: "name",
            messages: vec![
                Message {
                  id: "validation.length.max",
                  text: None,
                  args: vec!["3".to_string()]
                }
            ]
        }
    ], out);

    let result = validate! {
        "name" => name => [
            length::max(64)
        ],
        "description" => description => [
            length::max_if_present(255)
        ]
    };
    assert!(result.is_ok());