Update qadic_pow.
Update qadic_pow.

Removes some of the reductions mod p^N in qadic_pow.  This leads
to an improvement in qadic_teichmuller, too.

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