Move `StakingLedger` struct definition and implementation

[gpestana]

Follow up from #1484 (already merged).

The #1484 has implemented and refactored out some of the code from the StakingLedger into its own module (/src/ledger.rs). In order to keep the PR 1484 clean and with sane diffs, we kept half of the staking ledger related code in src/lib.rs. This issue consists of refactoring the code so that the StakingLedger definition and all impls are in the ledger module.

• Move the StakingLedger definition and implementation into /staking/src/lib.rs.

[joshua-fink]

Hi, how can I get involved? Looking for a cool open-source project to join :)

[omadoyeabraham]

@gpestana I would like to work on this, can I proceed?