Optionally optimize new iterators in debugging builds.

In the past, folks have complained about poor performance
in debugging builds. These new iterators are written such
that their complexity optimizes out, but there is still a
lot of complexity. On debugging builds without optimization,
this can really get in the way and start slowing things down.

In anticipation of this request, I'm adding a CMake option
VTK_ALWAYS_OPTIMIZE_ARRAY_ITERATORS which, as you might guess,
will always optimize array iterators on platforms that
support selective optimization in debug builds.