Implement pallet-bags-list and its interfaces with pallet-staking

[emostov]

closes #9440

polkadot companion paritytech/polkadot#3413

Notes for reviewers

Below is a table describing how an extrinsic would affect an account (relative to SortedListProvider and CounterForNominators) given its role in staking:

extrinsic	Chilled	Nominator	Validator
chill, chill_other	nothing, nothing	on_remove, dec counter	nothing, nothing
nominate	on_insert, inc counter	nothing, nothing	on_insert, inc counter
validate	nothing, nothing	on_remove, dec counter	nothing, nothing
bond_extra	nothing, nothing	on_update, nothing	nothing, nothing
rebond	nothing, nothing	on_update, nothing	nothing, nothing
unbond	nothing, nothing	on_update, nothing	nothing, nothing

TODO:

• update benchmarks of extrinsics that call do_rebag to force a rebag (relates to some work done in Force bag changes in relevant benchmarks (targets #9507) #9529)
• Check the benchmark values for snapshot creation. There are some tests that tell us how many snapshot items we can take in a 2 second block. Implement pallet-bags-list and its interfaces with pallet-staking #9507 (comment)
• Write a remote-externalities-based test that checks the state of polkadot and substrate, and populates the list. Same as testing the migration basically: Done in companion.
• Test migration

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• (seperate_pr) Make all staking tests work with and without bags-list. For now we use bags-list only, and we do this in a follow up, to prevent a single super big PR.
• rename and fix docs, avoid using voter in bags-list, it should be id and vote-weight. Ideally we could also use something like priority instead of vote-weight.
• (maybe+separate PR) Write a fuzzer/quick test that just bombards the SortedListProvider with random data and calls, and the internal state of the pallet should always remain correct. (Fuzzer for Pallet Bags List #9851)
• (seperate pr) weight refunds for when the operation does not affect nominators (and thus not the bags list)
• (separate PR) Allow the ability to reorder yourself within a bag.

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original description for #9081 below:

Problem: there are too many nominators, we can't keep all of them.
Solution: truncate the list by stake.

Problem: an attacker can kick out as many people as they can afford to; those people at the low end would need to re-nominate.
Solution: don't actually take them out of the nominator set, but compute the top N members at runtime.

Problem: computing the top N at runtime is expensive.
Solution: create many bags, each of which holds voters within a particular stake range. Each bag is a linked list, so it can be added to and removed from in O(1). We can then iterate over the bags in order and truncate anywhere we desire. Given a realistic distribution of stake, it doesn't matter where we stop; the final bag is the one with the lowest-staked individuals.

polkadot companion: paritytech/polkadot#3413

[emostov]

bot merge force

[ghost]

Trying merge.

[ghost]

Merge failed: "3 of 8 required status checks have not succeeded: 1 expected, 1 failing, and 1 pending."

[emostov]

bot merge

[ghost]

Trying merge.

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Merge failed: "Required status check "continuous-integration/gitlab-check-polkadot-companion-build" is failing."

[emostov]

bot merge

[ghost]

Trying merge.