pub fn p_minus(b: [b85; 3]) -> [b85; 3]Expand description
p - b for canonical b, computed as !b - 18 over 255 bits.
!b = 2^255 - 1 - b, so !b - 18 = 2^255 - 19 - b = p - b. Combinational.
For b = 0 the result is p itself, which is not canonical; callers
that need a canonical negation must special-case zero (see the machine’s
field_neg).