Show function names in error messages

Functions in Nix are anonymous, but if they're assigned to a
variable/attribute, we can use the variable/attribute name in error
messages, e.g.

while evaluating `concatMapStrings' at `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/strings.nix:18:25':
...

src/libexpr/parser.y

@@ -104,6 +104,7 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
             }
         }
     }
+    e->setName(attrPath.back());
 }