Allow labels starting with uppercase with OverloadedLabels

The following program

{-# LANGUAGE OverloadedLabels, DataKinds, FlexibleInstances,
  MultiParamTypeClasses #-}

import GHC.OverloadedLabels

instance IsLabel "Three" Int where
    fromLabel _ = 3

test :: Int
test = #Three

main :: IO ()
main = print test

fails to compile in ghc 8.0 with a parse error (while it works as expected if we replace "Three" -> "three"). This may be a conscious design decision, but if not I figured I would ask if it would be possible to allow such labels starting with uppercase letters.

I run into this when working on adding support for OverloadedLabels to the gobject-introspection bindings (autogenerated bindings for gtk, etc.), where it would be natural in a few places to write overloaded labels starting with a capital letter. Not hugely important, but sometimes aesthetically more pleasing (imho), and I am not aware of a good reason to forbid them. Perhaps there is one?

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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