recompute small sieve positions at each region, for the exceptional small primes.
recompute small sieve positions at each region, for the exceptional
small primes.

Those are rare, so one would expect that the cost is negligible.

We also get rid of small_sieve_skip_stride, and do the corresponding
work directly within the sieve functions.

Validation: tested on catrel on an rsa220 sample, jobs 25284 to 25315.
The addition "small sieve + resieve gains about 1% on average. However
we seem to lose some reports. Investigation needed.

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