ExtrOcamlZBigInt: Use shifts/masks to build and match positive values

[cpitclaudel]

This is particularly valuable for FMapPositive and other structures like stdpp's Pmap that walk a binary number's encoding, because walks can now be done without general-purpose division.

Micro-benchmark on OCaml 5.2.1+flambda:

  Name                              Time/Run     mWd/Run
  FMapPositive.find x100k (stock)    88.04ms   17_403_012w
  FMapPositive.find x100k (is_odd)   57.32ms            6w

[cpitclaudel]

I guess we could change other definitions as well:

Extract Constant Z.div_eucl => "(fun x y ->
    if Z.equal y Z.zero then (Z.zero, x)
    else let q = Z.fdiv x y in (q, Z.sub x (Z.mul y q)))".
  Extract Constant Z.div => "(fun x y ->
    if Z.equal y Z.zero then Z.zero else Z.fdiv x y)".
  Extract Constant Z.modulo => "(fun x y ->
    if Z.equal y Z.zero then x else Z.sub x (Z.mul y (Z.fdiv x y)))".

Micro-benchmarks suggest that these are 1.3-1.7x faster and use 3x less memory.