* same behavior for positive numbers
* generalisation for negative numbers, treating them as two complement
* improve algorithm, iterate once over the digits, manually perform two complement
* simplify mp_add_d, mp_sub_d
* functions are safe in case of a==c or b==c
* renamed mp_tc_div_2d to mp_signed_rsh (signed right shift)
The return type of mp_get_bit was imprecise (either mp_err or mp_bool),
therefore this function is deprecated in favor of s_mp_get_bit for now.
If we need s_mp_get_bit to be public, we should add it under a different
name. However since mp_set_bit is not available, I don't think there any
downstream users (ab)using mp_int as bitsets.
* In the default settings, a cutoff X can be modified at runtime
by adjusting the corresponding X_CUTOFF variable.
* Tunability of the library can be disabled at compile time
by defining the MP_FIXED_CUTOFFS macro.
* There is an additional file tommath_cutoffs.h, which defines
the default cutoffs. These can be adjusted manually or by the
autotuner.
