New public function mps_fmt_create_k creates an object format using the keyword argument interface.
Deprecate the format variant structures (mps_fmt_A_s, mps_fmt_B_s, mps_fmt_auto_header_s, mps_fmt_fixed_s) and the old interface (mps_fmt_create_A, mps_fmt_create_B, mps_fmt_create_auto_header, mps_fmt_create_fixed).