This PR refactors the staking ledger logic to encapsulate all reads and
mutations of `Ledger`, `Bonded`, `Payee` and stake locks within the
`StakingLedger` struct implementation.

With these changes, all the reads and mutations to the `Ledger`, `Payee`
and `Bonded` storage map should be done through the methods exposed by
StakingLedger to ensure the data and lock consistency of the operations.
The new introduced methods that mutate and read Ledger are:

- `ledger.update()`: inserts/updates a staking ledger in storage;
updates staking locks accordingly (and ledger.bond(), which is synthatic
sugar for ledger.update())
- `ledger.kill()`: removes all Bonded and StakingLedger related data for
a given ledger; updates staking locks accordingly;
`StakingLedger::get(account)`: queries both the `Bonded` and `Ledger`
storages and returns a `Option<StakingLedger>`. The pallet impl exposes
fn ledger(account) as synthatic sugar for `StakingLedger::get(account)`.

Retrieving a ledger with `StakingLedger::get()` can be done by providing
either a stash or controller account. The input must be wrapped in a
`StakingAccount` variant (Stash or Controller) which is treated
accordingly. This simplifies the caller API but will eventually be
deprecated once we completely get rid of the controller account in
staking. However, this refactor will help with the work necessary when
completely removing the controller.

Other goals:

- No logical changes have been introduced in this PR;
- No breaking changes or updates in wallets required;
- No new storage items or need to perform storage migrations;
- Centralise the changes to bonds and ledger updates to simplify the
OnStakingUpdate updates to the target list (related to
https://github.com/paritytech/polkadot-sdk/issues/443)

Note: it would be great to prevent or at least raise a warning if
`Ledger<T>`, `Payee<T>` and `Bonded<T>` storage types are accessed
outside the `StakingLedger` implementation. This PR should not get
blocked by that feature, but there's a tracking issue here
https://github.com/paritytech/polkadot-sdk/issues/149

Related and step towards
https://github.com/paritytech/polkadot-sdk/issues/443