Add point2<T> alias template for backward compatibility with Boost <=1.68.
Replace multiple point_t aliases of point<ptrdiff_t> with single defined
in point.hpp. The point_t is common used to represent dimensions.
Replace many uses of point<ptrdiff_t> with point_t.
Apply reformatting around point2 changes to respect the line length limit.
Follows up discussion in #154
