593: EitherOrBoth: Add or and or_else methods to simplify getting default values r=phimuemue a=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:
```rust
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:
```rust
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.


Co-authored-by: Carl Andersson <carl@carlandersson.net>