* 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.
* MP_USE_ENUMS enables enums
* Wc++-compat catches some implicit conversions if MP_USE_ENUMS is defined
* 100% backwards compatible API/ABI if MP_USE_ENUMS is not defined
