This commit merges in the changes from branches/arithmetic/refactor0

This commit merges in the changes from branches/arithmetic/refactor0
- Improved the checks in AssertLower and AssertUpper so that redundant bounds cause less work.
- Because of the above change, d_constantIntegerVariables now cannot have duplicate elements enqueued. This allows removing d_varsInDioSolver.
- Fix to an assertion in CDQueue.
- Implements a CDArithVarSet using a vector of booleans and CDList.
- Refactored ArithVar out of arith_utilities.h. Miscellaneous cleanup of arithmetic.