Decisions where not added in instantiations

   Improvements:
      - (ite c true false) is treated as c in patterns

   Fixes:
      - Why3's Q.compare was extracted to polymorphic compare instead of Q.compare

  Memorylimit with 20G reached on exemple of union.mlw

  The consequence was missing congruence closure

    terms can be grouped and scheduling of those decisions freezed

    Last decision and last effort made before contradiction should be useful so
    their priority is boosted.

    instantiate more rapidly reccursive definition.

  - Allow theories to define symbols to skip for trigger computation
  - Arithmetic skips <= < > >=

  - Makes Ite lazy
  - Partial fix for enumeration of reccursive ADT
  - Fix propagation of AST tests

  add some tests

Use more first class function for attaching event. The second part scheduled
keep the Dem.t type.