pub const PACKING_PAYS_AT: usize = 2;Expand description
How much smaller packing has to be before it is worth the shift and the mask on every read.
Two, so a column packs when the bits come to half the flat size or less. A column that would save a tenth stays flat, because a tenth of a column is not worth turning every read of it into arithmetic, and the whole argument for the form is that a narrow column saves most of itself.