Evaluating counter-examples again to faster cover other flags.

Replaced Seq[Identifier] with Option[Seq[Identifier]] to take into account transformed expressions without new identifiers.

Fix isDeterministic, introduce evalOrderSensitive

simplifyLets: Great simplifications, generalize LetTuple to LetPattern,
  use correct conditions.

SimplifierWithPaths now handles IfExpr, Or, And, Require, Implies.
Does not oversimplify LetPattern (simplifyLets does it soundly!)

simplifyTautologies: Redundant, removed

Simplifiers: rewrite to be more readable