Nth root modulo

Added functions
* Legendre, Jacobi, Kronecker symbols
* nthroot_mod and nthroot_mod_list
* powermod and powermod_list
* quotient_mod

Changed functions
* divides (now runs faster)
* mod
* fdiv_q renamed => quotient_f
This should be replaced with `if(is_a<Integer>(*b)) {` as well.