tuftool: do not use openssl to create a key

[Matthew James Briggs]

Let's not do this anymore:

tough/tuftool/src/root.rs

Line 276 in 807acd3

 let mut command = std::process::Command::new("openssl"); 

Surely this can be done in pure Rust instead. Maybe with this crate?

https://docs.rs/rsa/latest/rsa/