Uint<WIDTH> and Sint<WIDTH> arbitrary fixed-width types

[saxbophone]

Fixed-width unsigned and signed arithmetic types where the width they're fixed to is variable at compile-time, something like:

template <std::size_t BITS>
class Uint { ... };

template <std::size_t BITS>
class Sint { ... };

The exact naming will probably change, but it could be a nice idea.

Also, it'd be handy to specify some arithmetic operators as returning the smallest type big enough to store the largest possible result without over/underflow, so e.g. Uint<32> * Uint<32> -> Uint<64> and Uint<32> + Uint<32> = Uint<33>

If we're going to do that, we should definitely provide some convenient conversion functionality between the various different sizes of Uint<> and Sint<>...

Another efficiency idea could be to specialise instances of Uint<> / Sint<> for integer types smaller and equal to those supported natively on the target system, for efficiency. Doing so would require somewhat complicated template logic, but it should be looked into...

[saxbophone]

Implementation consideration: for addition and subtraction, maybe a method based on std::bitset<> and its bitwise operators could be used, for efficiency?

Yes, but the ripple-carry needs to be handled separately. Initial XOR and AND of input bits can be done with bitset's operators, and possibly the rest of the processing of the results after the ripple-carry is handled too.