Function to_case 

pub fn to_case<A: AsRef<str>>(case: Case) -> impl Fn(A) -> String

Converts a string to a specified case.

#[derive(Debug, Valust)]
struct Casing(
    #[trans(func(String => to_case(Case::Pascal)))]
    String
);
let case = RawCasing("snake_case".to_string());
let val = Casing::validate(case);
assert_eq!("SnakeCase", val.unwrap().0.as_str());