Implement summary.allEqual()

True if all elements of the iterable are equal.

```
funcion *allEqual<T>(data: Iterable<T> | Iterator<T>): boolean
```

Note: Consider maybe adding a strict parameter, or alternate method `allSame`.

Needs to be implemented:
* `summary.allEqual()`
* `summary.allEqualAsync()`
* `Stream.allEqual()`
* `AsyncStream.allEqual()`
