This reverts commit 24fa20e5.

Although these symmetric variants sometimes look "better", they
are really annoying and should IMHO never be used:

1.) For lemmas there is now a choice between >= and <=. Since there is
no longer a canonical choice, it is very easy to introduce a lot of
inconsistencies in statements of lemmas.

2.) For automation the situation becomes annoying, you have to built in
stuff for both >= and <=. That is very error-prone.

3.) For N and Z the notions x <= y and y >= x are not even convertible!
That means that done/by does not solve x <= y if you have y >= x and if
avoids you applying certain lemmas.

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.