Implement Checked{Add,Sub,Mul,Div}

[psimonyi]

When the T of a Ratio<T> supports checked arithmetic, the ratio should too.

I'm not really sure whether the checked_* functions should be #[inline] or not. They are in this patch because the non-checked operations are and the checked ones don't look that much bigger.

[psimonyi]

This is needed to support stepped iterators over Ratios:

extern crate num;

fn main() {
    let step = num::rational::Ratio::new(1, 3);
    for x in num::range_step(0.into(), 2.into(), step) {
        println!("{}", x);
    }
}

[psimonyi]

@cuviper ?

[cuviper]

Sorry for neglecting this -- it looks good!

bors r+

More generally, there's a lot of room to improve overflows. For example 1/128 + 1/128 overflows in u8, but if the denominator used LCM it could compute 2/128 -> 1/64. This is a problem for both add and checked_add here, so ideally we'd fix both -- only difference being in panic or None. But anyway, this is stuff for future work. I suppose I should file an issue.

[bors]

Build succeeded

• continuous-integration/travis-ci/push