Threadsafe ENV interaction

Closes #6
> The injection path felt a little weird by allowing as an argument to .modify, since ClimateControl::Environment feels like more a facade or decorator and something you'd probably not want to manage outside of Climate Control. Thoughts?

I think it's just a private class, used internally. The public API is just `ClimateControl.modify`, so as long as that remains simple, we can do whatever we think is most correct under the hood.