Show cycle backtraces when they happen

examples/mutex_cycle.rs

@@ -0,0 +1,26 @@
+//! Show what a crash looks like
+//!
+//! This shows what a traceback of a cycle detection looks like. It is expected to crash.
+use tracing_mutex::stdsync::Mutex;
+
+fn main() {
+    let a = Mutex::new(());
+    let b = Mutex::new(());
+    let c = Mutex::new(());
+
+    // Create an edge from a to b
+    {
+        let _a = a.lock();
+        let _b = b.lock();
+    }
+
+    // Create an edge from b to c
+    {
+        let _b = b.lock();
+        let _c = c.lock();
+    }
+
+    // Now crash by trying to add an edge from c to a
+    let _c = c.lock();
+    let _a = a.lock(); // This line will crash
+}