mp_expt_d: bring back pre 921be35779f7d71080ad85c27ed58671602d59b3 state

The implementation of the expt_d functionality is now implemented in the
mp_expt_d_ex() function.

The user can now choose between the old (more timing resistant) version
and the new version by modification of the parameter 'fast'.

mp_expt_d() defaults to the old version
mp_n_root: add mp_n_root_ex() with parameter 'fast' for mp_expt_d_ex()

This change is introduced to be able to choose the underlying
implementation of mp_expt_d_ex()

The implementation of the root_n functionality is now implemented in the
mp_n_root_ex() function.

The parameter 'fast' is just passed over to mp_expt_d_ex().

mp_n_root() defaults to the pre 921be35779f7d71080ad85c27ed58671602d59b3
implementation
demo: test both mp_n_root() implementations