feat: add checked_{add,sub} methods

[RRRadicalEdward]

This PR adds RGB::checked_add and RGB::checked_sub method. Do I need to add the methods for other pixels types (RGBA, etc)?

[kornelski]

There's no standard trait for checked operations, so unfortunately this will be pretty verbose and always incomplete.

I'm not sure if it's worth it over using .map().

[RRRadicalEdward]

I understand your point. Yes, there are no traits for checked operations, but I feel as if the RGB types behave like primitive types from std, most of which have checked_add/checked_sub functions so It makes sense for RGB<u8>, RGB<u16>, RGB<u32>, etc to have it as well whatever there's a trait or not for it because it mimics the std.