Migrate storage maps to use twox64_concat or blake2_128_concat

[shawntabrizi]

The various map pallets mostly use blake2_256 hashers and sometimes linked_maps and empty double maps. All linked maps should be removed. Double maps where the initial item is expected to be constant (e.g. pallet-session's NextKeys) should be altered to a simple map now. Hashers should be switched to either:

• blake2_128_concat in the case that the user is able to place map items whose keys are entirely at their discretion.
• twox64_concat in the case that the user cannot.

An example of the former would be the frame_system's Account storage item, since any address (even one without a known corresponding private key!) may be sent an existential deposit and therefore be created.

An example of the latter would be frame_system's BlockHash storage item, since its keys are limited to the block number integers. Users cannot populate it directly, nor can they influence the data.

A second example of the latter would be pallet-staking's Bonded item. Though this has a corresponding key type/value to frame_system's Account storage item, it has different semantics; no entry can be placed except through a valid signed transaction coming from the account of the key. As such its value is heavily constrained and cannot be selected at will (in general a public key would need to be known for the account and the distribution/correlation between public and secret keys are not such that a trie-imbalancing global griefing attack would be practical to execute).

Tasks

• Remove all the superfluous double-maps from Session & migrate.
• Replace all hasher(blake2_256)s with _concat hashers & provide either static or dynamic migration.

[kianenigma]

related: #4610