[zify] More aggressive application of saturation rules

The role of the `zify_saturate` tactic is to augment the goal with
positivity constraints.  The premisses were previously obtained from
the context. If they are not present, we instantiate the saturation
lemma anyway.

Also,
- Remove saturation rules for Z.mul, the reasoning is performed by lia/nia
- Run zify_saturate after zify_to_euclidean_division_equations
- Better lemma for Z.power
- Ensure that lemma are generated once

Co-authored-by:  Andrej Dudenhefner  <mrhaandi>

Closes #12184, #11656

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