feat(moving_targets): add new exercise for rust exams

subjects/moving_targets/README.md

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+## moving_targets
+
+### Instructions
+
+You will have a linked list of `Target` named `Field`.
+
+You will handle recursive types and ownership and implement the following **associated functions**:
+
+- `new`: which will initialize the `Field` with `head` set to `None`.
+- `push`: which receives a `Target` and add it as a `Node` at the head of the list.
+- `pop`: which returns the last added `Target` wrapped in an `Option` and removes it from the list.
+- `peek`: which returns the last added `Target` as a reference wrapped in an `Option` but do not removes it from the list.
+- `peek_mut`: which returns the last added `Target` as a mutable reference wrapped in an `Option` and also do not removes it from the list.
+
+You must also implement a type named `Link`. This will be the connection between the `Field` and `Target` structures. This will be a recursion type, and it must point to `None` if there is no `Target` to point to.
+
+### Expected Functions and structures
+
+```rust
+pub struct Field {
+    head: Link,
+}
+
+type Link = // To be implemented
+
+struct Node {
+    elem: Target,
+    next: Link,
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct Target {
+    pub size: u32,
+    pub xp: u32,
+}
+impl Field {
+    pub fn new() -> Self {}
+    pub fn push(&mut self, target: Target) {}
+    pub fn pop(&mut self) -> Option<Target> {}
+    pub fn peek(&self) -> Option<&Target> {}
+    pub fn peek_mut(&mut self) -> Option<&mut Target> {}
+}
+```
+
+### Usage
+
+Here is a program to test your function:
+
+```rust
+use moving_targets::*;
+
+fn main() {
+    let mut field = Field::new();
+    
+    println!("{:?}", field.pop());
+    field.push(Target { size: 12, xp: 2 });
+    println!("{:?}", *field.peek().unwrap());
+    field.push(Target { size: 24, xp: 4 });
+    println!("{:?}", field.pop());
+    let last_target = field.peek_mut().unwrap();
+    *last_target = Target { size: 2, xp: 0 };
+    println!("{:?}", field.pop());
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+None
+Target { size: 12, xp: 2 }
+Some(Target { size: 24, xp: 4 })
+Some(Target { size: 2, xp: 0 })
+$
+```
+
+### Notions
+
+- [Box\<T\>](https://doc.rust-lang.org/book/ch15-01-box.html)
+- [Linked lists in Rust](https://rust-unofficial.github.io/too-many-lists/index.html)