Blanket trait impls for Result

[jtempest]

Following the structural model, users should be able to give epsilon variants that match the different enum branches:

assert_float_eq!(Ok(0.1), Ok(0.2), abs <= Ok(0.1)); // true
assert_float_eq!(Ok(0.1), Err(0.2), abs <= Ok(0.1)); // false
assert_float_eq!(Err(0.1), Err(0.2), abs <= Ok(0.1)); // false, epsilon is a different variant
assert_float_eq!(Err(0.1), Err(0.2), abs <= Err(0.1)); // true

// chaining allows multiple variants to be compared
assert_float_eq!(a, b, abs <= Ok(0.1), Err <= (0.1)); 

There is also the question of what to do where enums have branches that aren't floating point values. This would require a solution to https://github.com/jtempest/float_eq-rs/issues/10. It's likely that the common case of Result would be T of some float value and E of a non-float, but it's plausible that the opposite could be true or both could be float comparable. In this case, it might be nice to fall back on PartialEq - which might require specialisation or the implementation of default associated types in traits.

[jtempest]

Closed, will be replaced by an issue with more in depth discussion of what to do with enum/Result.