Distribution.Compat.Error

[Ross Paterson]

To re-iterate an old suggestion: how about using a custom data type like

	data ParseResult a = ParseFailed PError | ParseOk a

instead of Either for parse results, thus removing the dependency
on mtl, which will help with bootstrapping.  I know there is
Distribution.Compat.Error, but that defines a Monad instance for Either,
so if one uses that there'll be trouble later if some program includes
both Cabal and mtl.