Fungibles: migrate Democracy pallet

[pgherveou]

Partial for #226

The (Reservable/Lockable)Currency traits are deprecated after this paritytech/substrate#12951 in favour of the fungible traits.

A few things have changed:

• ReservableCurrency::(reserve/unreserve) becomes fungible::MutateHold::(hold/release)
• LockableCurrency::(extend_lock/remove_lock/set_lock) becomes fungible::MutateFreeze::(extend_freeze/thaw/set_freeze)

Holds and Freezes

Holds are roughly analagous to reserves, but are now explicitly designed to be infallibly slashed. They do not contribute to the ED but do require a provider reference, removing any possibility of account reference counting from being problematic for a slash. They are also always named, ensuring different holds do not accidentally slash each other's balances.

Freezes are essentially the same as locks, except that they overlap with Holds. Since Holds are designed to be infallibly slashed, this means that any logic using a Freeze must handle the possibility of the frozen amount being reduced, potentially to zero. A permissionless function should be provided in order to allow bookkeeping to be updated in this instance.

Both Holds and Freezes require an identifier which is configurable and is expected to be an enum aggregated across all pallet instances of the runtime.

In the Pallet Democracy context we define HoldReason::Proposal and FreezeReason::Proposal.

/// A reason for this pallet placing a hold on funds.
#[pallet::composite_enum]
pub enum HoldReason {
  /// Funds are held when a proposal is submitted and are released when the proposal is
  /// tabled. If the proposal is vetoed and blacklisted, the deposit is slashed.
  Proposal,
}

/// A reason for this pallet placing a freeze on funds.
#[pallet::composite_enum]
pub enum FreezeReason {
  /// Funds are frozen upon casting a vote. They are subsequently unfrozen either
  /// when the vote is cancelled or after the specified lock period has elapsed (see
  /// [`conviction::Conviction`]).
  Vote,
}

Upgrade notes

The pallet_democracy::Config trait now requires new type RuntimeHoldReason and RuntimeFreezeReason:

impl pallet_democracy::Config for Runtime {
-  type Currency = pallet_balances::Pallet<Self>;
+  type Fungible = Balances;
+  type RuntimeHoldReason = RuntimeHoldReason;
+  type RuntimeFreezeReason = RuntimeFreezeReason;
}

Also, construct_runtime needs pallet_democracy::HoldReason and : pallet_democracy::FreezeReason:

construct_runtime!(
	pub enum Runtime
	{
		...
		Democracy: pallet_democracy::{Pallet, Call, Storage, Config<T>, Event<T>, HoldReason, FreezeReason },
		...
	}
)

Migration

A (multi-block) migration is provided with this PR, which will convert all existing locks to holds and freezes.

[pgherveou]

We may want to adapt treasury to accept slashes from pallet_democracy here...

[bkchr]

@pgherveou do you plan to finish this?

[pgherveou]

@pgherveou do you plan to finish this?

I think it's ready for review
But I will rebase it today against #4989 as it should simply the migration code, although maybe mbm is an overkill for this pallet 🤔