Instead, I separate it into a total function and a predicate
describe whether the action is allowed or not. This has some
advantages:

* It is much easier to deal with total functions and predicates
  in Coq than with functions into option.
* Already existing functions do not need to be wrapped. Instead,
  when using a local update you end up with a sensible side
  condition as a Coq Prop.
* The definition of local updates (and all CMRA instances) no
  longer depend on option.

This way we avoid many one-off indexes and no longer need special cases for
index 0 in many definitions. For example, the definition of the distance
relation on option and excl has become much easier. Also, uPreds no longer need
to hold at index 0.

In order to make this change possible, we had to change the notions of
"contractive functions" and "chains" slightly.

Thanks to Aleš Bizjak and Amin Timany for suggesting this change and to help
with the proofs.

Now notations are pretty printed in the same way as they are parsed.
Before "let x := e1 in e2" was notation for "(fun x => e2) e1",
resulting in overlapping notations for the same thing.

It is now slightly below implication. In order to do this, I had to change
the notation from P ={E1,E2}=> Q to P >{E1,E2}=> Q because the prefer ={n
is already used at level 70 for the distance of the metric.