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.

Cargo.toml

@@ -50,7 +50,6 @@ serde = { version = "1", features = ["derive"] }
 env_logger = "0.9"
 rustls = { version = "0.20", features = ["dangerous_configuration"] }
 rustls-pemfile = { version = "1.0" }
-mbedtls = { version = "0.8.1" }
 
 [[example]]
 name = "cureq"