Support checked ops.

[RinChanNOWWW]

OrderedFloat only implement these ops now:

impl_ordered_float_binop! {Add, add, AddAssign, add_assign}
impl_ordered_float_binop! {Sub, sub, SubAssign, sub_assign}
impl_ordered_float_binop! {Mul, mul, MulAssign, mul_assign}
impl_ordered_float_binop! {Div, div, DivAssign, div_assign}
impl_ordered_float_binop! {Rem, rem, RemAssign, rem_assign}

It's useful to support num_traits::ops::checked::{CheckedAdd, CheckedSub, ...}.