Are celpy Environments thread safe?

Hello!

I was wondering if a celpy Environment instance could be shared amongst threads, or if we should create a new instance when we want to create a runner for a new expression.

I tried to look through the docs and source code but didn't find any reference to this.

One thing I've noticed though is that, when we call the program method, the code is doing an intermediate assignment to an instance variable of the environment, before returning, which I believe can be problematic in terms of thread preemption.
```
self.runnable = runner_class(self, expr, functions)
return self.runnable
```

Nonetheless I might have missed something and would like to get your feedback on this use case.

Thank you very much!