Use heap allocations for longedges to reduce memory usage of prepared graphs

[nmandery]

https://github.com/nnethercote/perf-book

use std::collections::HashSet;

struct LongEdge {
    in_edge: u64,
    out_edge: u64,
    edge_path: Vec<u64>,
    cell_lookup: HashSet<u64>,
}

struct OwnedEdgeValueBoxed {
    weight: f32,
    longedge: Option<Box<(LongEdge, f32)>>,
}

struct OwnedEdgeValue {
    weight: f32,
    longedge: Option<(LongEdge, f32)>,
}

fn main() {
    dbg!(std::mem::size_of::<OwnedEdgeValue>());
    dbg!(std::mem::size_of::<OwnedEdgeValueBoxed>());
}

Output:

    Finished dev [unoptimized + debuginfo] target(s) in 0.58s
     Running `target/debug/playground`
[src/main.rs:22] std::mem::size_of::<OwnedEdgeValue>() = 104
[src/main.rs:23] std::mem::size_of::<OwnedEdgeValueBoxed>() = 16