Mutex & Safe access to FatDriver

[malandrakisgeo]

Hello!

I am a total n00b in Rust. I like your project, and I thought I could somewhat improve my Rust skills by fixing one of your TODOs. Hence this simple mutex for safe access to structures.

Feel free to diss my code. I'll try to make this mutex somewhat more complex and safe, as soon as time allows it.

[mrgian]

Ehi @malandrakisgeo great job!
This is something I should have done sooner but never got into it.
The goal is to make Felix completely memory safe in the future, and this is the first step into it!

Thank you!