Accept arbitrary rust code as case argument

[la10736]

As #18 pointed out is too cleaver use Unwrap("some rust code"). We can do better and parse arbitrary rust code in procedural macro attributes

Case like follow should be parsed:

case(42, -42, pippo("pluto"), Vec::new(), String::from(r#"prrr"#), 
            {
                let mut sum=0;
                for i in 1..3 {
                    sum += i;
                }
                sum
            }, vec![1,2,3])