Update Exercises Directory Names to Reflect Order

exercises/21_macros/README.md

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+# Macros
+
+Rust's macro system is very powerful, but also kind of difficult to wrap your
+head around. We're not going to teach you how to write your own fully-featured
+macros. Instead, we'll show you how to use and create them.
+
+If you'd like to learn more about writing your own macros, the
+[macrokata](https://github.com/tfpk/macrokata) project has a similar style
+of exercises to Rustlings, but is all about learning to write Macros.
+
+## Further information
+
+- [Macros](https://doc.rust-lang.org/book/ch19-06-macros.html)
+- [The Little Book of Rust Macros](https://veykril.github.io/tlborm/)

exercises/21_macros/macros1.rs

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+// macros1.rs
+//
+// Execute `rustlings hint macros1` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+}
+
+fn main() {
+    my_macro();
+}

exercises/21_macros/macros2.rs

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+// macros2.rs
+//
+// Execute `rustlings hint macros2` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    my_macro!();
+}
+
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+}

exercises/21_macros/macros3.rs

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+// macros3.rs
+//
+// Make me compile, without taking the macro out of the module!
+//
+// Execute `rustlings hint macros3` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+mod macros {
+    macro_rules! my_macro {
+        () => {
+            println!("Check out my macro!");
+        };
+    }
+}
+
+fn main() {
+    my_macro!();
+}

exercises/21_macros/macros4.rs

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+// macros4.rs
+//
+// Execute `rustlings hint macros4` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+#[rustfmt::skip]
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    }
+    ($val:expr) => {
+        println!("Look at this other macro: {}", $val);
+    }
+}
+
+fn main() {
+    my_macro!();
+    my_macro!(7777);
+}