Make build work (#546)

The mbedtls example has caused problem in the main build a number of
times. By making it a standalone `cargo new --bin`, we can keep it in
the source tree as a good example but avoid having it break the main
build.

Also, fix some clippy lints.

examples/mbedtls/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "mbedtls-example"
+version = "0.0.1"
+edition = "2021"
+
+
+[dependencies]
+mbedtls = { version = "0.8.1" }