HListPP: A preprocessor for HList labelable labels

A preprocessor that replaces `longDescriptiveName with

hLens' (Label :: Label "longDescriptiveName")

Use with ghc flags -F -pgmF HListPP. A file using HListPP could start with something like

❴-# OPTIONS_GHC -F -pgmF HListPP #-❵ -- change braces to ascii (haddock doesn't allow literal comments in files)
❴-# LANGUAGE DataKinds #-❵
import Data.HList.CommonMain
import Control.Lens

Then the labels can then be used in expressions with types like:

`x :: Lens (Record S) (Record T) a b
`x :: Prism (Variant S) (Variant T) a b
`a .==. "x" .*.
   `b .==. "y" .*. emptyRecord :: Record [ Tagged "a" String, Tagged "b" String ]

where S is a type-level list containing a Tagged "x" a and T is a type-level list containing a Tagged "x" b.

Note that `labels is expanded to something that takes up about 26 more columns. To avoid issues with layout, put a newline after keywords that introduce layout, at least if there are `labels in the same row. In other words:

`character.`position.`x %= \ x -> let
     msg = "a really long message about x>0"
     pos = x - 5
   in if pos < 0
         then error msg
         else pos

Instead of

`character.`position.`x %= \ x -> let msg = "a really long message about x>0"
                                      pos = x - 5
                           in if pos < 0
                               then error msg
                               else pos