Fix parser for projection by labels

[Gabriella439]

… to not consume a trailing comma OUTSIDE of the braces.

Fixes #2088

The two alternatives for the labels parser already support
trailing commas INSIDE of the braces. The buggy line appears to
have been left behind by mistake.

This also left-factors the field projection parser, which is a
slight parsing performance improvement I noticed along the way.

[Gabriella439]

Hmmm, doctests appear to be reliably failing for the appveyor build recently. I see the same issue on #2087

C:\projects\dhall-haskell\dhall\src/Dhall/Deriving.hs:281: failure in expression `:{
font :: Decoder Font
font =
  union
    (  (Arial         <$ constructor "Arial"           unit)
    <> (ComicSans     <$ constructor "Comic Sans"      unit)
    <> (Helvetica     <$ constructor "Helvetica"       unit)
    <> (TimesNewRoman <$ constructor "Times New Roman" unit)
    )
:}'
expected: 
 but got: "<interactive>: addLibrarySearchPath: C:\\Users\\appveyor\\AppData\\Local\\Programs\\stack\\x86_64-windows\\msys2-20180531\\mingw64\\bin (Win32 error 3): The system cannot find the path specified.\r"
          "<interactive>: addLibrarySearchPath: C:\\Users\\appveyor\\AppData\\Local\\Programs\\stack\\x86_64-windows\\msys2-20180531\\mingw64\\lib (Win32 error 3): The system cannot find the path specified.\r"
          "<interactive>: addLibrarySearchPath: C:\\Users\\appveyor\\AppData\\Local\\Programs\\stack\\x86_64-windows\\msys2-20180531\\mingw64\\bin (Win32 error 3): The system cannot find the path specified.\r"
          "<interactive>: addLibrarySearchPath: C:\\Users\\appveyoExamples: 253  Tried: 250  Errors: 0  Failures: 1

[Gabriella439]

I suspect that this is due to a bad build cache on Appveyor, so I'm going to attempt to follow the instructions here:

https://www.appveyor.com/docs/build-cache/#cleaning-up-cache

[Gabriella439]

Yeah, it turns out that it was an issue with the cache. For future reference, I managed to clear it out by running:

$ curl --verbose -X DELETE --header "Authorization: Bearer ${TOKEN}" https://ci.appveyor.com/api/projects/Gabriel439/dhall-haskell/buildcache

… and then restarted the Appveyor build, which now passes.