Introduce rb_proc_new and test it

[tinco]

I tried making it accept more than one arguments, but failed. If I make it be variadic, and then transmute multi-argument function references to that, the second argument will still always be nil. I don't know enough about variadic argument functions to understand why this would happen.

I'm not sure if this is good to merge in as it is, since a proper variadic implementation might not be compatible with consumers of this API.

Also, I hope you don't mind I made a test for it, it helped find the right signatures and finding out if all the values were being set correctly. Also I think it's nice if there's examples of how to use these functions in here, because there sure aren't any in the Ruby codebase.

[steveklabnik]

Also, I hope you don't mind I made a test for it, it helped find the right signatures and finding out if all the values were being set correctly. Also I think it's nice if there's examples of how to use these functions in here, because there sure aren't any in the Ruby codebase.

Not at all! And I agree 100%.

I'm not mega mega familiar with the details here, so I'll give this a real review at some point, but I can't right now. Thank you for the PR!

[tinco]

This PR was not 100% correct, after reading a bunch of Ruby code and some help from the rust IRC channel I fully understand how it works now. I've got working code in my branch now, I'll clean it up and push it to this PR when I get some time.