Rename Update into Reaction

[maldins46]

In our modelization, we refer to Update as a function that takes a State as an input and returns an eventually modified State (State => State). We referred to this also as an Effect in the past, as stated in #8.

But in concrete terms, the Update arises primarily from an Action, possibly combined with some other conditions that favor this Update to be triggered.

For these reasons, I think It could be more intuitive to change the keyword from Update to Reaction: the Update is effectively something that should be done to react to an Action, so I think it is a more correct term.