Add State snapshotting cheats support

[pkhry]

Description

see title.

[smiasojed]

why do we need to rollback transaction before run?

[pkhry]

just in case if i missed it anywhere, ie table tests or else

[pkhry]

but given that setup happens outside a transaction it's changes are not being rolled back

[smiasojed]

why you have not decided to just copy backend like this:

pub fn save_revive_snapshot(snapshot_id: U256) {
    TEST_EXTERNALITIES.with_borrow_mut(|test_ext| {
        let backend = test_ext.as_backend();
        REVIVE_SNAPSHOTS.with_borrow_mut(|snapshots| {
            snapshots.insert(snapshot_id, backend);
        });
    });
}

And revert it like this

pub fn revert_revive_snapshot(snapshot_id: U256) -> bool {
    REVIVE_SNAPSHOTS.with_borrow_mut(|snapshots| {
        if let Some(backend) = snapshots.get(&snapshot_id).cloned() {
            let mut test_externalities = ExtBuilder::default().build();
            let mut backend_copy = backend;
            std::mem::swap(&mut test_externalities.backend, &mut backend_copy);
            TEST_EXTERNALITIES.set(test_externalities);
            true
        } else {
            false
        }
    })
}

Of course this implementation is based on old code base, but when I have implemented it the changes were very minimal.

[pkhry]

transaction commit and rollback are much more faster

[smiasojed]

why? It is mem swap/copy, Rollbacks you are doing in loops. Do you have any numbers?
I see that as_backend could be slow, is there an option to use into_raw_snapshot

[pkhry]

yeah, i'll post the comparative changes today. for fuzz tests for example executor.clone was very much noticeable in the .trace output compared to transaction_start and transaction_rollback

[pkhry]

why? It is mem swap/copy, Rollbacks you are doing in loops. Do you have any numbers?
I see that as_backend could be slow, is there an option to use into_raw_snapshot

it was even slower than as_backend(), in the thread local impl but i'll add benches today to an issue and tag you so that it's more visible