Improve qadic_inv in the case len = 1.
Improve qadic_inv in the case len = 1.

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