feat(prime_checker): add new rust exercise

subjects/prime_checker/README.md

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+## prime_checker
+
+### Instructions
+
+Create a **function** `prime_checker` that takes an `u32` and check if it is a prime number.
+
+The result will be `None` if the argument is less or equal one otherwise, it will return the result with the prime number inside if the number is prime, or it will return an `enum` `PrimeErr` if the `u32` is not prime.
+
+The `enum` `PrimeErr` will be `Even` if the number is even or `Divider(u32)` where the `u32` is the smaller divider of the number if it is not prime.
+
+### Expected Function and structure
+
+```rust
+#[derive(PartialEq, Eq, Debug)]
+pub enum PrimeErr {
+    Even,
+    Divider(u32),
+}
+
+pub fn prime_checker(nb: u32) -> Option<Result<u32, PrimeErr>> {}
+```
+
+### Usage
+
+Here is a program to test your function:
+
+```rust
+use prime_checker::*;
+
+fn main() {
+    println!(
+        "Is {} prime? {}",
+        2,
+        match prime_checker(2) {
+            Some(res) => {
+                match res {
+                    Ok(_) => "Yes, it is!",
+                    Err(_) => "No, it is not!",
+                }
+            }
+            None => "Less than 2",
+        }
+    );
+    println!(
+        "Is {} prime? {}",
+        14,
+        match prime_checker(14) {
+            Some(res) => {
+                match res {
+                    Ok(_) => "Yes, it is!",
+                    Err(_) => "No, it is not!",
+                }
+            }
+            None => "Less than 2",
+        }
+    );
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+Is 2 prime? Yes, it is!
+Is 14 prime? No, it is not!
+$
+```