Functor and Applicative instances for ExprArr

```
    q = (,,,,)
     <$> pure (Nothing :: Maybe (Wire Int))
     <*> (Just <$> constant (Post.author      post))
     <*> (Just <$> constant (Post.createdTime post))
     <*> (Just <$> constant (Post.title       post))
     <*> (Just <$> constant (Post.content     post))
```

<=>

```
  q = proc () -> do
          author  <- constant (Post.author post) -< ()
          time <- constant (Post.createdTime post) -< ()
          title <- constant (Post.title post) -< ()
          content <- constant (Post.content post) -< ()
          returnA -< (Nothing :: Maybe (Wire Int), Just author, Just time, Just title, Just content)
```