Safe and sane multi-item storage removal (#11490)

* Fix overlay prefix removal result

* Second part of the overlay prefix removal fix.

* Report only items deleted from storage in clear_prefix

* Fix kill_prefix

* Formatting

* Remove unused code

* Fixes

* Fixes

* Introduce clear_prefix host function v3

* Formatting

* Use v2 for now

* Fixes

* Formatting

* Docs

* Child prefix removal should also hide v3 for now

* Fixes

* Fixes

* Formatting

* Fixes

* apply_to_keys_whle takes start_at

* apply_to_keys_whle takes start_at

* apply_to_keys_whle takes start_at

* Cursor API; force limits

* Use unsafe deprecated functions

* Formatting

* Fixes

* Grumbles

* Fixes

* Docs

* Some nitpicks 🙈

* Update primitives/externalities/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Formatting

* Fixes

* cargo fmt

* Fixes

* Update primitives/io/src/lib.rs

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

* Formatting

* Fixes

Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

substrate/frame/staking/src/slashing.rs

@@ -557,7 +557,9 @@ impl<'a, T: 'a + Config> Drop for InspectingSpans<'a, T> {
 
 /// Clear slashing metadata for an obsolete era.
 pub(crate) fn clear_era_metadata<T: Config>(obsolete_era: EraIndex) {
+	#[allow(deprecated)]
 	<Pallet<T> as Store>::ValidatorSlashInEra::remove_prefix(&obsolete_era, None);
+	#[allow(deprecated)]
 	<Pallet<T> as Store>::NominatorSlashInEra::remove_prefix(&obsolete_era, None);
 }