To reduce the number of true casts

[petrochenkov]

@leonardo

3. If not already present to introduce in the standard library a safe conversion function like "to". (This is even more important in Rust because generally the compiler catches integrals overflows in debug mode. So using "as" is a very blunt tool that goes against such higher safety).

There's an RFC about this:

RFC: Checked integer conversions

With that RFC and existing From/Into impls you could write lossless and lossy casts as

let a = 10u8;
let b = a.into(): u16; // Lossless
let c = b.cast(): u8; // Lossy, catches overflows in debug mode

Unfortunately, From/Into impls for usize and isize met some resistance and weren't implemented yet.

(Speaking of D, its parentheses elision is great! With it these casts would look like b.cast: u8)