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 921be35779
implementation
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
