Implement SecretType::into_secret

[IvanUkhov]

It would be helpful to have a method to get the ownership of the underlying string back. What do you think?

[ramosbugs]

Hey @IvanUkhov,

This sounds reasonable. The Rust idiom I often see is an accessor method called inner() that returns a reference and another method called into_inner() that takes ownership and returns the inner value. Since the secret types currently have an accessor called secret(), it would probably make sense to call the new method into_secret() instead of implementing the From trait. This naming also helps remind callers that they're dealing with a sensitive value they should be careful with, which may help keep these secrets out of logs or other undesirable sinks.

[IvanUkhov]

Clear. Done in #272.