Width of the coefficient column. One unit wider than the decimal’s coefficient so
that each element is 56 bytes, a whole number of u64 words. The indexed byte
store pads every column to whole words and cannot decode an element width that
does not divide that padding.
One column per raw part. The coefficient units use Vec<[u16; N]> directly rather
than the array’s own columnar container, which would add per-element offsets to a
fixed-width value.