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!

Issue 提到了 Environment.program 和 self.runnable 的赋值，但没有指定文件或测试。首先跟踪该实现，并在文档和源代码中搜索现有的线程安全保证；如果没有覆盖率，则测试共享使用和按线程使用 Environment 的情况。完成的标准是给出关于支持哪种用法的明确答复，并通过文档或回归测试体现出来。

由索引模型根据 Issue 内容生成。