feat(data/zmod): lemmas about totient and zmod (#2158)
* feat(data/zmod): lemmas about totient and zmod
* docstring
* Changes based on Johan's comments
* fix build
* subsingleton (units(zmod 2))
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
