add "Fast constant-time gcd computation and modular inversion"

[haraldschilly]

Sage is used in the paper https://eprint.iacr.org/2019/266, which will appear in TCHES 2019. Beyond the paper itself, the paper web page

https://gcd.cr.yp.to/papers.html

has various test scripts using Sage.

bibtex:

@article{bernstein2019fast,
  title={Fast constant-time gcd computation and modular inversion},
  author={Bernstein, Daniel J and Yang, Bo-Yin},
  journal={IACR Transactions on Cryptographic Hardware and Embedded Systems},
  pages={340--398},
  year={2019}
}