Update and modernify dependencies

[AlexTMjugador]

While I was working on other PRs, I noticed that this crate depended on slightly older versions of crates, so I took the initiative to update all of the dependencies :smile:

This PR is not meant to alter any functionality of this crate. Some assorted highlights about it include:

• I've replaced the dirs dependency with the functionally-equivalent home. This decision was driven by the fact you already know that its latest version transitively pulls a dependency on a copylefted crate, and the repeated refusal of upstream to do something about it. home is maintained by the official Rust language team, making it more likely to keep its licensing in check for the foreseeable future.
• I've swapped out lazy_static with once_cell. The latter eliminates the need for the cumbersome macro syntax and aligns more closely with what the Rust standard library may eventually stabilize. Thus, it's currently considered the best choice.
• I got rid of an unnecessary tokio feature for library usage.
• I executed cargo deny and updated its configuration file accordingly.
• Unnecessary wrapping of global variables with RefCell and Arc was eliminated.

[emk]

Ah, thank you! Unfortunately, it may be a while before I get back to credentials; we only use it in a tiny handful of rarely updated projects these days. I will try to look at this then.

[emk]

Actually, I see you have submitted several very nice PRs! Thank you.

I'll bump this up my TODO list but it still may not be right away. I do really appreciate the quality PRs, though.

[AlexTMjugador]

No worries, I'm happy to read you appreciate the PRs! Please tackle them at the pace you feel most comfortable with :+1: