Derive Debug where possible

[qsantos]

I mostly wanted to dbg! the server's Session, but I took the opportunity to add #[derive(Debug)] wherever I could.

[Eugeny]

A lot of these are empty structs and structs with security relevant fields that should not end up in some forgotten dbg! output. I could merge a handmade Debug impl with just the relevant fields for Session but not this.

[qsantos]

I have excluded the private keys from the Debug implementation of the server Session, but I probably missed some other secrets.

[Eugeny]

LGTM! @all-contributors add @qsantos for code

[allcontributors]

@Eugeny

I've put up a pull request to add @qsantos! 🎉