Specialize div for 128/256-bit uint with 32/64-bit limbs (generalize to 4 limbs)

[ckormanyos]

The popular case of uint128 is not yet specialized or specifically optimized.

Consider to potentially specialize mul/div for 128-bit uint with 32-bit limbs

[ckormanyos]

See also #2 and this link to 128-bit division

[ckormanyos]

This has been split from to handle mul and div separately, with mul in #30.

[ckormanyos]

This will be generalized to specialization to 4 limbs of any limb type, 8, 16, 32 or 64 bit.

[ckormanyos]

This leads to too much code, in the sense of actually too many independent lines of code. Close with no intent to follow up.