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.
