Expose a method to return the memory usage of all dynamic atoms.

[jdm]

We need to perform a manual calculation because https://github.com/bholley/malloc_size_of_derive/ does not contain a MallocSizeOf implementation for [...].

Changes done

[SimonSapin]

@jdm I’ve pushed a rewrite of this PR on top of recent changes, without using derive, and without using recursion to walk the linked list

Could you check it with your Servo branch?

[jdm]

It is currently impossible to test the changes because something about the upgrade to the 0.10 completely breaks page loading so the HTML content appears in the page: https://github.com/servo/servo/compare/main...jdm:servo:measure-dynamic?expand=1

Edit: aha, it's the phf update to 0.14 that messes everything up. When I revert that change, this appears to work with my changes to report it in about:memory.

[SimonSapin]

We do have this impl, does Servo not enable it?

#[cfg(feature = "malloc_size_of")]
impl<Static: StaticAtomSet> malloc_size_of::MallocSizeOf for Atom<Static> {
    fn size_of(&self, _ops: &mut malloc_size_of::MallocSizeOfOps) -> usize {
        0
    }
}

[SimonSapin]

When I revert that change, this appears to work with my changes to report it in about:memory.

Sounds good, thanks for testing!