warn on casting enum to numeric value

[durka]

This code

#![warn(clippy)]
#![warn(clippy_pedantic)]

fn it_works() {
    enum Foo { Bar = 300 }

    let x = 300i32 as u8;
    let y = Foo::Bar as u8;
}

hits a warning on x but not on y, even though both are truncated. Probably one of the cast_* lints should trigger whenever a repr(rust) enum is as-casted (or if e.g. a repr(i32) is casted to a u8), optionally checking to see if the enum actually has numeric variants that would cause problems.