OneOf Generator

This was a little bit tricky to keep stateless because when you want to shrink to an earlier generator here we have to determine which is the generator used to get the current element you want to shrink

I think this could work, but check it out :heart: 
Makes a lot of sense in the case of disjoint domains such as `oneOf($letters, $integers, $booleans)`: in that case picking the Generator that contains the current value is simple and is the correct thing to do