Added pollard brent implementation in ulong extras.
Added pollard brent implementation in ulong extras.

jpflori/flint2,dsroche/flint2,jpflori/flint2,wbhart/flint2,jpflori/flint2,dsroche/flint2,dsroche/flint2,dsroche/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2,jpflori/flint2,wbhart/flint2,wbhart/flint2