Both the field and scalar modulus can be written in signed{30,62} notation
with one or more zero limbs. Make use of this in the update_de function to
avoid a few wide multiplications when that is the case.
This doesn't appear to be a win in the 32-bit implementation, so only
do it for the 64-bit one.
This commit adds functions to verify and compare numbers in signed{30,62} notation,
and uses that to do more extensive bounds checking on various variables in the modinv
code.
This adds a long comment explaining the algorithm and implementation choices by building
it up step by step in Python.
Comments in the code are also reworked/added, with references to the long explanation.
