Use is_sorted, merge, copy from std (#199)

Use is_sorted, merge, copy from std (#199)

Previously, we were checking whether we should use is_sorted
from std or __gnu_cxx. With C++11, std::is_sorted is
guaranteed to exist. This commit changes
arith/normal_form.{h,cpp} to always use std::is_sorted and
also removes the custom implementations for merge and copy by
using the std implementations instead.