Add logging exercise

exercise/logging/src/lib.rs

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+// 1. Bring the macros error, warn, info, debug, and trace into scope from the log package with a
+// `use` statement.
+//
+// - You should be able to run `cargo build --lib` successfully after each step.
+//
+// Hint: You may need to update Cargo.toml first
+
+#[derive(Debug)]
+pub struct Frog {
+    energy: u8,
+    sleeping: bool,
+}
+
+impl Frog {
+    pub fn new() -> Self {
+        // 2. Use debug!() to log "A new Frog has been created"
+        Default::default()
+    }
+    pub fn hop(&mut self) {
+        // 3. Use info!() to log that a Frog hopped, and how much energy is left
+        self.energy -= 1;
+        if self.energy == 0 {
+            // 4. Use warn!() to warn that the frog will go to sleep since he ran out of energy
+            self.sleep();
+        }
+    }
+    // info, error
+    pub fn sleep(&mut self) {
+        if !self.sleeping {
+            // 5. Use error!() to log a (non-fatal) error stating that the Frog is already asleep
+            self.sleeping = true;
+        }
+    }
+}
+
+impl Default for Frog {
+    fn default() -> Self {
+        // 6. Use trace!() to log that a default value was generated, with the debug representation
+        Frog {
+            energy: 5,
+            sleeping: false,
+        }
+    }
+}

exercise/logging/src/main.rs

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+// START IN lib.rs!!!
+
+use frogger::Frog;
+
+// You did 1-6 in lib.rs already, right?
+//
+// 7. Bring env_logger into scope. You might need to update Cargo.toml, first.
+
+fn main() {
+    // 8. Initialize env_logger using the init() function
+
+    // 9. Run this program with `cargo run` and take a look at the default output.
+    // - Now run it again with an explicit log level, like `RUST_LOG=info cargo run`
+    // - ...and `RUST_LOG=trace cargo run`
+    let mut skippy = Frog::new();
+    skippy.hop();
+    skippy.hop();
+    skippy.hop();
+    skippy.hop();
+    skippy.hop();
+    skippy.sleep();
+
+    // Challenge: Go back to lib.rs and set the `target: ` argument for each logging call to be the
+    // path to the function.  For example, `Frog::new`
+}