Upgrade all dependencies, eliminate duplicate `heapless`, eliminate `unsafe` from examples.

[ColinFinck]

embedded-io, rustls, and all RustCrypto crates are under heavy development, with many breaking changes even compared to their latest released versions. Hence, I didn't stop half-way, but went for the latest bleeding-edge prerelease versions of these crates. This PR should likely be merged once all dependencies are available as final versions.

Anyway, I'd be happy if you could already have a look and give some early feedback.

CC @bugadani @lulf @newAM