Multi-module projects

This proposal is to have projects that consist of more than one module, where the main module imports others. The UI, I suppose, would look like tabs within the editor area. The goal would be to keep this as simple as possible for users who don't want it. I am not interested in making the UI more complex for beginners in order to support a few advanced students; but it would be good to have a reasonable path for those advanced students.

This also makes Haskell-mode much more appealing as a general-purpose tool for learning Haskell.

该 issue 没有指定任何文件、测试或入口点。首先梳理 editor 当前如何处理单个 module，以及 UI 如何在不增加初学者复杂度的情况下表示已导入的 modules。完成的标准是：主 module 可以导入其他 modules，并且高级用户拥有清晰的多 module workflow。

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