This is so that we can write and read things like [Void], or (Expr Void)
- something I'm actually doing in Morte.

https://github.com/Gabriel439/Haskell-Morte-Library/pull/26

The reader always fails if you actually try to get an instance of it.
Trying to access a void value should not happen for well-formed
instances, for example:

    *Data.Binary Data.Void> decode $ encode ([] :: [Void]) :: [Void]
    []