Various small changes.

* Define the standard strict order on pre orders.
* Prove that this strict order is well founded for finite sets and finite maps.
  We also provide some utilities to compute with well founded recursion.
* Improve the "simplify_option_equality" tactic to handle more cases.
* Axiomatize finiteness of finite maps by translation to lists, instead of by
  them having a finite domain.
* Prove many additional properties of finite maps.
* Add many functions and theorems on lists, including: permutations, resize,
  filter, ...