feat(solver)!: Make Problem use builder pattern

[eviltak]

Changes the Problem input struct (introduced in #54) to use the builder pattern.

This allows the soft_requirements field to be an Iterator rather than a Vec, which prevents having to unnecessarily clone the SolvableIds passed to soft_requirements (which in some cases could be on the order of hundreds of thousands). Also, this prevents adding a new input argument to Solver::solve (i.e. a new field in Problem) from being a breaking change.

The C++ bindings remain unchanged and rely on using a simple resolvo_cpp::Problem struct as before; they do not expose the builder pattern.

[baszalmstra]

Thanks as always! :)