Implement very simple digraph with cycle detection

Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "tracing-mutex"
+version = "0.1.0"
+authors = ["Bert Peters <bert@bertptrs.nl>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]