Get new sieve code working.  More big.Int -> bigint.
Cleanup.
Fix comments.
Move bigint library to its own directory.
Use new bigint package.
Rename file now that it is in its own package.
Rename constructor.
Reorganize file.
Use scratch space so Mod64 doesn't have to allocate.
Clean up a bit.
Move primes to its own package.
Use prime powers in new sieve
Get new qs using sieve working
Rearrange sievesmooth to use callback to send results
Add single bigprime matching
Some comments, gofmt.
Fix up precondition comment.
Fix sieve test.