cis194: Homework seven not running under GHC 8.10.2

Executing `runhaskell StringBufEditor.hs` from Yorgey's seventh homework with GHC 8.10.2 results in:

```
Editor.hs:38:22: error:
• No instance for (Applicative (Editor b))
arising from the 'deriving' clause of a data type declaration
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
• When deriving the instance for (Monad (Editor b))
|
38 | deriving (Functor, Monad, MonadIO, MonadState (b,Int))
|
```

A fix could be provided or at least a remark included in the cis194 section.