Refactor SrcLoc and SrcSpan
The "Unhelpful" cases are now in a separate type. This allows us to improve various things, e.g.: * Most of the panic's in SrcLoc are now gone * The Lexer now works with RealSrcSpans rather than SrcSpans, i.e. it knows that it has real locations and thus can assume that the line number etc really exists * Some of the more suspicious cases are no longer necessary, e.g. we no longer need this case in advanceSrcLoc: advanceSrcLoc loc _ = loc -- Better than nothing More improvements can probably be made, e.g. tick locations can probably use RealSrcSpans too.