Correct location annotation for parenthesized expressions

At the moment parentheses are not included in the location annotation
for nix expressions. This changes the code to include any parentheses in
the SrcSpan of the underlying expression. Information which is difficult
to recover otherwise.

src/Nix/Parser.hs

@@ -58,6 +58,7 @@ import           Data.Char                      ( isAlpha
                                                 , isSpace
                                                 )
 import           Data.Data                      ( Data(..) )
+import           Data.Fix                       ( Fix(..) )
 import           Data.Functor
 import           Data.Functor.Identity
 import           Data.HashSet                   ( HashSet )
@@ -195,8 +196,11 @@ nixBool =
 nixNull :: Parser NExprLoc
 nixNull = annotateLocation1 (mkNullF <$ reserved "null" <?> "null")
 
+-- | 'nixTopLevelForm' returns an expression annotated with a source position,
+-- however this position doesn't include the parsed parentheses, so remove the
+-- "inner" location annotateion and annotate again, including the parentheses.
 nixParens :: Parser NExprLoc
-nixParens = parens nixToplevelForm <?> "parens"
+nixParens = annotateLocation1 (stripAnn . unFix <$> (parens nixToplevelForm <?> "parens"))
 
 nixList :: Parser NExprLoc
 nixList = annotateLocation1 (brackets (NList <$> many nixTerm) <?> "list")