Function plan 

pub fn plan(
    gens: &[GenerationId],
    protect: &[GenerationId],
    keep: usize,
    max_bytes: u64,
    measure: impl FnMut(&[GenerationId]) -> u64,
) -> GcPlan

Plan a collection. gens are the generations on disk, protect the I3 set; measure gives the size of a set of generations’ combined closures.