a possible solution for unlocking the GVL

[Maaarcocr]

Hi! I've seen in past issues that possibly allowing the GVL to be unlocked for rust functions would be something that magnus may do in the future. I've recently prototyped a way to do so in https://github.com/Maaarcocr/lucchetto

I was wondering if there would be any interest in mainlining that approach here, as I think it can be pretty useful to be able to decide if you want to hold the gvl lock or not.

I know that there are some safety concerns around unlocking the GVL and for that I made a small marker trait which is implemented for types that I deem to be safe to be passed in/out of functions that do not hold the GVL. Now this does not entirely disallow calling into ruby, but it's a nice first step.

For now it only works for free functions, as they are easier to make safe.

Anyway, I would be happy to scratch that repo and port the code here if there is interest! I think it works nicely.

I also recognise that the code is very young and it makes use of unsafe quite a bit, but I'm sure with some additional time spent on it we could make sure it's all good.