Apply updates recursively

[dapplion]

Motivation

Current main branch has a foot gun for nested List/Vectors. The code below will panic on tree_hash_root because l_sub still has pending updates. Ethereum consensus does have many nested complex types that involve mutability, but it would be nice to remove the possibility of runtime errors if there's not much overhead.

let mut l = List::<List<u64, U16>, U16>::default();

l.push(<_>::default()).unwrap();
let l_sub = l.get_mut(0).unwrap();
l_sub.push(1).unwrap();
let v = l_sub.get_mut(0).unwrap();
assert_eq!(*v, 1);
// l_sub.apply_updates().unwrap(); <<- Needs this line to not panic
l.apply_updates().unwrap();
l.tree_hash_root()

Impl

This PR introduces a new trait PendingUpdates that all values of List / Vector must implement. I'll take suggestions for a better name, since this will be very prevalent in the Lighthouse codebase.

@michaelsproul comments that the compiler may optimize an apply implementation inside of the for_each_mut loop away. However I'm not sure if that's the case with a Result involved + the current impl. Open to feedback.

Next

• derive macro to implement PendingUpdates for structs