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## scytale_cipher
### Instructions
Create a **function** which decode a scytale cipher (also known as spartan cipher).
In practice, it is represented by a strip wrapped around a cylinder. The message is written across the loops of the strip (not along the strip). The message becomes *coded* if the radius of the cylinder changes, or the strip is removed from the cylinder.
Your function will receive a `String` representing the ciphered message, and a `u32` representing the number of letters by turn of the strip around the cylinder.
> If the ciphered message is empty or the letters per turn are 0 the function will return `None`.
### Example
**letters_per_turn 2:** `"scytale Code"` -> `"sec yCtoadle"`
```console
--------------------------------
|s| |c| |y| |t| |a| |l|
|e| | | |C| |o| |d| |e|
--------------------------------
```
**letters_per_turn 4:** `"scytale Code"` -> `"steoca dylCe"`
```console
------------------------------------------
|s| |c| |y|
|t| |a| |l|
|e| | | |C|
|o| |d| |e|
------------------------------------------
```
### Expected Functions
```rust
pub fn scytale_decoder(s: String, letters_per_turn: u32) -> Option<String> {
}
```
### Usage
Here is a program to test your function
```rust
fn main() {
println!("\"sec yCtoadle\" size=2 -> {:?}",
scytale_decoder("sec yCtoadle".to_string(), 2));
println!("\"steoca dylCe\" size=4 -> {:?}",
scytale_decoder("steoca dylCe".to_string(), 4));
}
```
And its output
```console
$ cargo run
"sec yCtoadle" size=2 -> Some("scytale Code")
"steoca dylCe" size=4 -> Some("scytale Code")
$
```