Use correct type for second template argument of `std::array`

Using unsigned long instead of size_t only works on architectures where the two types are identical, but not on, for example, i386. On architectures where the types differ, the generic version would be used instead of the specialization for std::array.

Bug-Debian: https://bugs.debian.org/860666

enabled an automatic merge when the pipeline for a722b3bc succeeds

mentioned in commit 1318e0e3

merged

mentioned in commit 81a4487b

mentioned in merge request !91 (merged)