feat: added new exercises for generics

exercises/generics/README.md

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+### Generics
+
+In this section you'll learn about saving yourself many lines of code with generics!
+
+### Book Sections
+
+- [Generic Data Types](https://doc.rust-lang.org/stable/book/ch10-01-syntax.html)

exercises/generics/generics1.rs

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+// This shopping list program isn't compiling! 
+// Use your knowledge of generics to fix it.
+
+// I AM NOT DONE
+
+fn main() {
+    let mut shopping_list: Vec<?> = Vec::new();
+    shopping_list.push("milk");
+}
+

exercises/generics/generics2.rs

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+// This powerful wrapper provides the ability to store a positive integer value.
+// Rewrite it using generics so that it supports wrapping ANY type.
+struct Wrapper<u32> {
+    value: u32
+}
+
+impl<u32> Wrapper<u32> {
+    pub fn new(value: u32) -> Self {
+        Wrapper { value }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn store_u32_in_wrapper() {
+        assert_eq!(Wrapper::new(42).value,  42);
+    }
+
+    #[test]
+    fn store_str_in_wrapper() {
+        // TODO: Delete this assert and uncomment the one  below once you have  finished the exercise.
+        assert!(false);
+        // assert_eq!(Wrapper::new("Foo").value, "Foo");
+    }
+}

exercises/generics/generics3.rs

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+// An imaginary magical school has a new report card generation system written in rust!
+// Currently the system only supports creating report cards where the student's grade 
+// is represented numerically (e.g. 1.0 -> 5.5). However, the school also issues alphabetical grades
+// (A+ -> F-) and needs to be able to print both types of report card!
+
+// Make the necessary code changes to support alphabetical report cards, thereby making the second
+// test pass.
+
+pub struct ReportCard {
+    pub grade: f32,
+    pub student_name: String,
+    pub student_age: u8,
+}
+
+impl ReportCard {
+    pub fn print(&self) -> String {
+        format!("{} ({}) - achieved a grade of {}", &self.student_name, &self.student_age, &self.grade)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generate_numeric_report_card() {
+        let report_card = ReportCard {
+            grade: 2.1, 
+            student_name: "Tom Wriggle".to_string(), 
+            student_age: 12,
+        };
+        assert_eq!(report_card.print(), "Tom Wriggle (12) - achieved a grade of 2.1");
+    }
+
+    #[test]
+    fn generate_alphabetic_report_card() {
+        // TODO: Make sure to change the grade here after you finish the exercise.
+        let report_card = ReportCard {
+            grade: 2.1, 
+            student_name: "Gary Plotter".to_string(), 
+            student_age: 11,
+        };
+        assert_eq!(report_card.print(), "Gary Plotter (11) - achieved a grade of A+");
+    }
+}