Add a warning for casts with loss of precision

[bfops]

e.g. this code should probably generate a warning:

fn main() {
    let i: u16 = 257;
    println!("{}", i as u8);
}

(Sorry if this has been mentioned elsewhere - I didn't see it when searching for "cast warning")

[japaric]

(For the general case, you could use num::cast which returns None if the operation under/overflows (it does a check at runtime).)

As for the lint request, I think that's a dupe of #14165

[bfops]

Ah good to know! Thanks!

I don't think it's quite a dupe because that issue only refers to literals - it should maybe be generalized?

[steveklabnik]

closing as dup of #5477