Implement num-trait's saturating operations

[Dietr1ch]

num_traits defines a family of Saturating operations that come out handy from time to time.

• SaturatingAdd
• SaturatingSub
• SaturatingMul

I think that num_traits is already an optional dependency, so I guess this might need a feature flag.

There's also a new rust feature adding a saturating_add in the works, but I don't think that the num_traits traits will become irrelevant before core Rust support lands.

• saturating_add
• saturating_sub

[mbrubeck]

I think that num_traits is already an optional dependency, so I guess this might need a feature flag.

No feature flag needed; num_traits is a non-optional dependency.

[Dietr1ch]

I took a stab at it and noticed that floats are not currently Saturating in num_traits, but AFAIK only something like a BigInteger isn't saturating.

I filed rust-num/num-traits#361 to implement Saturating for floating point numbers in the first place.