Add boolean functions to check whether the extension is enabled or disabled

Sometimes it's helpful to have a function:

```haskell
isEnabled :: Extension -> Set OnOffExtension -> Bool
```

That checks whether there's `On` for the particular given extension.

Does it sound useful? For now, I propose not to think about the fact that some extensions can imply other extensions, this is another whole story..

Start by locating the definitions of Extension and OnOffExtension and the existing set-based API. Add checks for whether a given extension is represented as On or Off, and verify both cases in the relevant tests; done means the proposed boolean functions work without addressing implied extensions.

Written by the indexing model from the issue text.