Update Exercises Directory Names to Reflect Order

exercises/02_functions/README.md

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+# Functions
+
+Here, you'll learn how to write functions and how the Rust compiler can help you debug errors even
+in more complex code.
+
+## Further information
+
+- [How Functions Work](https://doc.rust-lang.org/book/ch03-03-how-functions-work.html)

exercises/02_functions/functions1.rs

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+// functions1.rs
+//
+// Execute `rustlings hint functions1` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    call_me();
+}

exercises/02_functions/functions2.rs

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+// functions2.rs
+//
+// Execute `rustlings hint functions2` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    call_me(3);
+}
+
+fn call_me(num:) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}

exercises/02_functions/functions3.rs

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+// functions3.rs
+//
+// Execute `rustlings hint functions3` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    call_me();
+}
+
+fn call_me(num: u32) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}

exercises/02_functions/functions4.rs

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+// functions4.rs
+//
+// This store is having a sale where if the price is an even number, you get 10
+// Rustbucks off, but if it's an odd number, it's 3 Rustbucks off. (Don't worry
+// about the function bodies themselves, we're only interested in the signatures
+// for now. If anything, this is a good way to peek ahead to future exercises!)
+//
+// Execute `rustlings hint functions4` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    let original_price = 51;
+    println!("Your sale price is {}", sale_price(original_price));
+}
+
+fn sale_price(price: i32) -> {
+    if is_even(price) {
+        price - 10
+    } else {
+        price - 3
+    }
+}
+
+fn is_even(num: i32) -> bool {
+    num % 2 == 0
+}

exercises/02_functions/functions5.rs

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+// functions5.rs
+//
+// Execute `rustlings hint functions5` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    let answer = square(3);
+    println!("The square of 3 is {}", answer);
+}
+
+fn square(num: i32) -> i32 {
+    num * num;
+}