add functions for number theory, including (self-initializing) quadratic sieve

[TimoKellerMath]

I'm currently implementing basic functions for number theory (done: Miller–Rabin test, Jacobi symbol, Shanks–Tonelli algorithm). My goal is to write an implementation of the SIQS for integer factorization, to be included in num-bigint.

[TimoKellerMath]

Before I create a pull request for my first additions: There are some pull requests that haven't been merged yet, but relevant to my code, e.g. primality testing #33. (When) will this happen? @cuviper If some of these changes would have been merged, I would adapt my code accordingly.