Flat argument list instead of nested tuples for closures in Validation methods

[haraldmaida]

Currently the closures in the methods Validation::and_then and Validation::map` get passed nested tuples which can become really ugly with more and more arguments.

We should find a way to flatten the values collected so far by the Validation type, so that the closures either get a list of arguments passed or one flat tuple with all the values.

This will be a breaking change in the API, but I think the advantage outweighs any pain with changing existing implementations.