[Staking] Noop refactor to prep pallet for currency fungible migration (#5399)

This is a no-op refactor of staking pallet to move all `T::Currency` api
calls under one module.

A followup PR (https://github.com/paritytech/polkadot-sdk/pull/5501)
will implement the Currency <> Fungible migration for the pallet.

Introduces the new `asset` module that centralizes all interaction with
`T::Currency`. This is an attempt to try minimising staking logic
changes to minimal parts of the codebase.

## Things of note
- `T::Currency::free_balance` in current implementation includes both
staked (locked) and liquid tokens (kinda sounds wrong to call it free
then). This PR renames it to `stakeable_balance` (any better name
suggestions?). With #5501, this will become `free balance that can be
held/staked` + `already held/staked balance`.