## string_permutation

### Instructions

Define the **function** `is_permutation`, that returns `true` if the string `s1` is a permutation of `s2`.

`s1` is a permutation of `s2` if all the elements in `s1` appear the same number of times in `s2`, and all the characters in `s1` appear in `s2` even if they are in different order.

### Expected Function

```rust
pub fn is_permutation(s1: &str, s2: &str) -> bool {
}
```

### Usage

Here is a program to test your function.

```rust
use string_permutation::*;

fn main() {
	let word = "thought";
	let word1 = "thougth";
	println!(
		"Is `{}` a permutation of `{}`? = {}",
		word,
		word1,
		is_permutation(word, word1)
	);
}
```

And its output

```console
$ cargo run
Is `thought` a permutation of `thougth`? = true
$
```

### Notions

- [hash maps](https://doc.rust-lang.org/book/ch08-03-hash-maps.html)