Quadratic sieve factoring

[oscardssmith]

After being heavily nerd-sniped by @s-celles, here is a quadratic sieve implementation. It's already a good bit faster than my ECM implementation in #104 for semi-primes (and there still is some relatively low hanging fruit for further optimization, specifically looking for x^2 - m*N and using multiple polynomials). Unfortunately for factoring big numbers with lots of factors it's fairly slow (so probably needs combining with ECM with a low bound)

julia> x = prod(nextprime(big(2)^30, i) for i in 1:2)
1152921515344265237

julia> @time Primes.QSfactor(x)
  0.006857 seconds (25.64 k allocations: 1.795 MiB)

julia> x = prod(nextprime(big(2)^50, i) for i in 1:2)
1267650600228402790082356974917

julia> @time Primes.QSfactor(x)
  0.592893 seconds (353.31 k allocations: 55.101 MiB, 4.61% gc time)

julia> x = prod(nextprime(big(2)^30, i) for i in 1:4)
1329228037874877665163654221879315219

julia> @time Primes.QSfactor(x)
  5.168209 seconds (1.19 M allocations: 313.809 MiB, 0.56% gc time)