Function flatten`Const`

flatten<T>(): (src: Iterable) => Iterable
Flattens a single level of nested iterables into a lazy iterable of their elements.

Curried version of _flatten.
Type Parameters
- T
  Element type produced by each inner iterable.
Returns (src: Iterable) => Iterable
A deferred iterable yielding every element from each inner iterable in order.
Example
```
const values = [..._flatten([[1, 2], [3, 4]])];
console.log(values); // [1, 2, 3, 4]
```
or using the curried version:
```
const values = pipeInto(
  [["a"], ["b", "c"]],
  flatten()
);
console.log([...values]); // ["a", "b", "c"]
```