feat(drop_the_blog): add exercise for rust exams

subjects/drop_the_blog/README.md

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+## drop_the_blog
+
+### Instructions
+
+Define the following structures:
+
+- `Blog`: containing:
+  - `drops`: that will save the number of dropped articles.
+  - `states`: that will save the state of multiple articles. If the article is not dropped, the state will be `false`, otherwise it will be `true`.
+- `Article`: containing:
+  - `id` as `usize`.
+  - `body`: as `String`.
+  - `parent`: a link to the structure `Blog`. (Tip: this should be a reference).
+
+You'll need to also add the following associated functions to the structures:
+
+- `Blog`:
+  - `new`: that creates an empty blog.
+  - `new_article`: that receives a `String` for the body and returns a tuple with the `id` and a new `Article`.
+  - `is_dropped`: that receives an `id` and returns a `bool` that indicates the state of the article.
+  - `new_id`: which returns a `usize` representing the length of the `states` vector. (Which is also the first available id).
+  - `add_drop`: which is **called by the `Drop` trait**. It will receive an `id` that will be used to change the state of the article. If the state of that article is `true` then it will panic with the message `"X is already dropped"`, where `X` represents the `id`). Otherwise it should change the state to `true` and increment the `drops` field by 1.
+
+- `Article`:
+  - `new`: that initializes a new article.
+  - `discard`: that drops the article.
+
+> You must implement the `Drop` trait for the `Article` structure. In this trait you must call the function `add_drop` so that the state of the article changes.
+
+### Expected Functions
+
+```rust
+use std::cell::{RefCell, Cell};
+
+#[derive(Debug, Default, Clone, Eq, PartialEq)]
+pub struct Blog {
+    pub drops: Cell<usize>,
+    pub states: RefCell<Vec<bool>>
+}
+
+impl Blog {
+    pub fn new() -> Blog {}
+    pub fn new_article(&self, body: String) -> (usize, Article) {}
+    pub fn new_id(&self) -> usize {}
+    pub fn is_dropped(&self, id: usize) -> bool {}
+    pub fn add_drop(&self, id: usize) {}
+}
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct Article<'a> {
+    // expected public fields
+}
+
+impl<'a> Article<'a> {
+    pub fn new(id: usize, body: String, blog: &'a Blog) -> Article {}
+    pub fn discard(self) {}
+}
+```
+
+### Usage
+
+Here is a program to test your function,
+
+```rust
+use drop_the_blog::*;
+use std::rc::Rc;
+
+fn main() {
+    let blog = Blog::new();
+    let (id, article) = blog.new_article(String::from("Winter is coming"));
+    let (id1, article1) = blog.new_article(String::from("The story of the universe"));
+
+    article.discard();
+
+    println!("{:?}", (blog.is_dropped(id), id, &blog.drops));
+
+    article1.discard();
+    println!("{:?}", (blog.is_dropped(id1), id1, &blog.drops));
+
+    let (id2, article2) = blog.new_article(String::from("How to cook 101"));
+    let article2 = Rc::new(article2);
+    let article2_clone = article2.clone();
+
+    drop(article2_clone);
+
+    println!("{:?}",
+      (blog.is_dropped(id2), id2, &blog.drops, Rc::strong_count(&article2)));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+(true, 0, Cell { value: 1 })
+(true, 1, Cell { value: 2 })
+(false, 2, Cell { value: 2 }, 1)
+$
+```
+
+### Notions
+
+- [Trait std::ops::Drop](https://doc.bccnsoft.com/docs/rust-1.36.0-docs-html/std/ops/trait.Drop.html)
+- [Struct std::cell::RefCell](https://doc.rust-lang.org/std/cell/struct.RefCell.html)
+- [Interior Mutability](https://doc.rust-lang.org/book/ch15-05-interior-mutability.html)