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
I assume there is no harm in using `} else if (l == 0) {` here instead of line 989 and 990 right?