Explicit resource management (using keyword)

https://github.com/tc39/proposal-explicit-resource-management

The ultimate goal is to support the `using` keyword. 

```js
import {temporaryFiles} from "using-temporary-files";

const api = using temporaryFiles();

// api.add(...)
// api.path(...)
// etc. 
```

It won't take much effort to implement. I just haven't gotten around to it yet.  

The existing promise-based API will remain.