Improve efficiency of is_smax and is_smin

[Robbepop]

The current implementation of Bitvec::is_smax and Bitvec::is_smin is suboptimal because there is no native implementation for it in apint and we have to emulate the logic for it in stevia_bitvec. To improve this we could try implementing these (or similar) checks in apint since they might be reasonable to add there because of the already existing constructors unsigned_max_value etc.