GHC's CPP and Cabal's unlit

[Alistair Bayley]

> To be honest, it's not clear to me that we need any distinction at all
> since people can just swap the order of the comments:
>
> -- some more comments about x
> --
> -- | A description of x
>
> x = ...

Yes, this is not a good motivating example, just an example. However,
a good motivating example is named chunks, which we use in Takusen's
Database.Enumerator module. The chucks are all placed at the end of
the file. A given chunk may contain many paragraphs, but it must
consist of a single comment block. Separate chunks must be in separate
comment blocks (I assume; I haven't actually tested this).

Alistair