Add right biased `Either` type to encode short circution instead of error

[ivenmarquardt]

Since Except always returns an Error oibject in case of exception, it cannot be used for short circuiting, for instance in foldM. Hence the more general Either is needed.

Either will be implemented as a sum type, whereas Except uses Error to indicate an exception and is transparent otherwise.

[ivenmarquardt]

done.