Add a test showing that mod_inverse is incorrect.
Add a test showing that mod_inverse is incorrect.

mod_inverse looks like it's incorrectly implemented.

This function is also not needed, `mod_inverse(a, b)` can be replaced with
`modexp_lr_k_ary(a, b-2, b)` (as long as b is prime, which is the case
in DSA), making the overall code shorter.

rrottmann/pydsa