[mpz_poly.cpp] implement 2-level Karatsuba for degree-3 squaring
[mpz_poly.cpp] implement 2-level Karatsuba for degree-3 squaring

mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs