Reduce memory footprint of SubsetParameterization

Change-Id: If113cb4696d5aef3e50eed01fba7a3d4143b7ec8

include/ceres/local_parameterization.h

diff --git a/include/ceres/local_parameterization.h b/include/ceres/local_parameterization.h
index 46a3931..67633de 100644
--- a/include/ceres/local_parameterization.h
+++ b/include/ceres/local_parameterization.h
@@ -192,7 +192,7 @@
 
  private:
   const int local_size_;
-  std::vector<int> constancy_mask_;
+  std::vector<char> constancy_mask_;
 };
 
 // Plus(x, delta) = [cos(|delta|), sin(|delta|) delta / |delta|] * x