Refactor pallet-xcm to use fungible traits

[bkontur]

Part of: #226
Relates to: paritytech/substrate#12951

/// A lockable currency.
// TODO: We should really use a trait which can handle multiple currencies.
type Currency: LockableCurrency<Self::AccountId, Moment = BlockNumberFor<Self>>;

TODO/Open questions

• Replace LockableCurrency with fungible and freeze traits
• Fix xcm_executor::traits::Enact implementations that use T::Currency
• Check if migration is needed
• Add try_state impl that checks LockedFungibles correctness with type Currency state (is it even possible)?
• If we want to support multiple currencies, should we address it here or as a separate issue afterward?

[acatangiu]

Duplicate of #7226