EitherOrBoth: Add or and or_else methods to simplify getting default values

[carl-anders]

Introducing or and or_else methods to EitherOrBoth

This feature reduces the amount of code required to get specific default values when using EitherOrBoth.
An example of the current way of using zip_longest with custom default values:

let a = (0..=4).into_iter();
let b = (0..=2).into_iter();
let c = a.zip_longest(b).map(|e| match e {
    EitherOrBoth::Both(l, r) => (l, r),
    EitherOrBoth::Left(l) => (l, 2),
    EitherOrBoth::Right(r) => (4, r),
});
// c will now contain an iterator with (0,0), (1,1), (2,2), (3,2), (4,2).

An example with the proposed or method:

let a = (0..=4).into_iter();
let b = (0..=2).into_iter();
let c = a.zip_longest(b).map(|e| e.or(4, 2));
// c will now contain an iterator with (0,0), (1,1), (2,2), (3,2), (4,2).

I have also included the or_else method which does the same but with closures.

Contribute questions

Include tests for your new feature, preferably a QuickCheck test

There are no tests for the other EitherOrBoth methods, so I was unsure how to place them. However, I have added DocTest's to both methods. These examples allows for easier to understand documentation, and also tests the validity of the methods.

For new features, please first consider filing a PR to rust-lang/rust

The EitherOrBoth struct does not exist in rust std library.

Concerns

The naming is slightly inconsistent when compared to rust's std::Option, considering the naming from there the added methods should be named unwrap_or and unwrap_or_else.
However, this would then become inconsistent with the existing method EitherOrBoth::or_default. Which is why I went with the chosen names.
I can change the method names if needed, but then we should consider changing or_default too.

P.S.

The CHANGELOG.md file has the text Add EitherOrBoth::or_default (#583). This number is wrong, it should be #538.

[phimuemue]

Thanks for this; a nice generalization of or_default! I think we can take this as it is.

bors r+

[bors]

Build succeeded:

• bors build finished