Dividing BigInts of the same limb count, where the quotient is one limb,
takes Knuth's quotient estimate from the top limbs as they read
normalized and settles it with one pass of a - q * b over the operands,
instead of copying both operands shifted and running algorithm D. The
remainder, when wanted, is that pass's result.
The limb loops for add, subtract, multiply by a limb, and multiply-add
and multiply-subtract by a limb process four limbs a pass, with the
carries in add-with-carry chains (__builtin_addcll and __builtin_subcll
where the compiler has them).
Chris Tate
ff70c33e38