Function components 

pub fn components(p: &Problem) -> Vec<Component>

Split a problem into independent components.

• Fixed variables never connect rules (they are constants).
• Rules with no free variable form their own Backend::Trivial component (checked for violation only).
• Free variables touched only by targets/stays form trivial components.
• A component is Backend::Linear when every row of every rule is affine in the free variables; otherwise the whole component is Backend::Numeric.