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.
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.