tommath.h contains declarations for the public part of the library.
tommath_private.h contains the functions which are private to ltm and
should not be exposed to the public.
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
