box: introduce template parameters for the sort order check

Since the sort order check is not required for keys without
descending parts it's decided to move the check to template,
so that the keys that has no descending parts don't pay for
the sort order check on each comparison.

Created proxy functions with increasing number of template
parameters to make the comparator selection code less
branchy.

`key_def_set_compare_func_for_func_index` has been renamed
to `key_def_set_compare_func_of_func_index` in order to
make the proxy-call fit in 80 lines.

NO_DOC=see previous commits
NO_TEST=see previous commits
NO_CHANGELOG=see previous commits