Add Proc coercions

[frewsxcv]

I'm working on a project where I want to pass a Proc from Ruby to Rust, and then Rust can invoke the Proc.

I think this should be possible? Not sure if there are any caveats. I found this resource which seems helpful:

https://clalance.blogspot.com/2011/01/writing-ruby-extensions-in-c-part-11.html

Time permitting, I can look into doing this - mainly opening this issue to hear if others have any concerns about this.