## edit_distance

### Instructions

Create a **function** named `edit_distance`, which calculates the minimum number of changes (insertions, deletions and/or substitutions) which are needed to transform the `source` string to the `target` string.

### Expected Function

```rust
pub fn edit_distance(source: &str, target: &str) -> usize {
}
```

### Usage

Here is a program to test your function.

```rust
use edit_distance::*;

fn main() {
	let source = "alignment";
	let target = "assignment";
	println!(
		"It's necessary to make {} change(s) to {}, to get {}",
		edit_distance(source, target),
		source,
		target
	);
}
```

And its output:

```console
$ cargo run
It's necessary to make 2 change(s) to alignment, to get assignment
$
```

### Notions

[Edit Distance (Wikipedia)](https://en.wikipedia.org/wiki/Edit_distance)