Use new multi-output hints to improve bit decomposition

[gbotrel]

Current bit decomposition extracts bits one by one in the solver (IthBit). Would now make sense to extract them all in the same hint call to avoid converting to big int & decomposing a field element in bits n times (n == nbBits).

[gbotrel]

https://github.com/ConsenSys/gnark/tree/hint-to-binary

Adds some changes to the hint API though (cc @ivokub ), holding off on that because:

On large circuits with many bit decompositions, the current hint (i-th bit) is called... millions of time 😅 . Part of the perf hit is actually on the big int thing; having a Constraint system + Hint system that's typed with field elements through generics (#172 ) will again change the API.