Move all examples to their own crates (#201)

This makes it much clearer which dependencies each example has.

examples/testing/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "example-testing"
+version = "0.1.0"
+edition = "2018"
+publish = false
+
+[dependencies]
+axum = { path = "../.." }
+tokio = { version = "1.0", features = ["full"] }
+tracing = "0.1"
+tracing-subscriber = "0.2"
+tower-http = { version = "0.1", features = ["trace"] }
+serde_json = "1.0"
+hyper = { version = "0.14", features = ["full"] }
+
+[dev-dependencies]
+tower = { version = "0.4", features = ["util"] }