New Trust region loop. 1. New TrustRegionMinimizer and basic tests for it. 2. New TrustRegionStrategy interface. 3. New LevenbergMarquardtStrategy and tests for it. 4. Updates to SolverImpl to reflect this. 5. Changes to Solver::Options and IterationSummary related to this. 6. Deleted levenberg_marquardt.cc/h/_test.cc Change-Id: I6c1d1a7c774f014856f9f26263a830aa886e1400