OneStep evaluator
Some fixes
Implements set operations in OneStepEval
Correct Typo
Add set operations support to the OneStepEvaluator
Begins undoStep support
Corrects MatchExpr case since ifExpr could appear in the expression to return
Corrects issues concerning guards in GuardedCase and refactors the MatchExpr case
Add PartiallyEvaluatedFunction to the trees in order to be able to print pre and post conditions from functions which had their args evaluated
Adds function invocation full support, match lacks only report of patterns in expression returned
Finishes match support