Speed up congruence solver

[Mangara]

While congruences can be quickly brute-forced for small n, some of the transformations of even quadratic equations with small coefficients generate congruences with n in the hundred-thousands. In these cases, it would be better to use a faster algorithm.

Even just factoring the modulus, solving the smaller congruences and combining the result may help.

These sources may be helpful:

• http://www.numbertheory.org/pdfs/quadratic_congruence.pdf
• https://www.johndcook.com/blog/quadratic_congruences/
• https://en.wikipedia.org/wiki/Tonelli%E2%80%93Shanks_algorithm

[Mangara]

Once this is done, we can stop skipping test equation 19 ( $42x^2 + 8xy + 15y^2 + 23x + 17y - 4915 = 0$ ).