A Reciprocal represents an integer division by any non-zero
u64. It replaces PartialReciprocal’s expression,
f(x) = (x + increment) * multiplier >> (64 + shift), where
the inner addition is a saturating add by 0 or 1, with
g(x) = (x * multiplier + summand) >> (64 + shift), where
both the multiplication and additions are in full 128 bit
arithmetic. This additional work lets us handle all the cases,
including divisions by 1 and by u64::MAX.