Introduce VestingCurrency trait (#4257)

* Introduce `VestingCurrency` trait

* Return error if account already has vesting schedule

substrate/frame/support/src/traits.rs

@@ -602,6 +602,29 @@ pub trait LockableCurrency<AccountId>: Currency<AccountId> {
 	);
 }
 
+/// A currency whose accounts can have balances which vest over time.
+pub trait VestingCurrency<AccountId>: Currency<AccountId> {
+	/// The quantity used to denote time; usually just a `BlockNumber`.
+	type Moment;
+
+	/// Get the amount that is currently being vested and cannot be transferred out of this account.
+	fn vesting_balance(who: &AccountId) -> Self::Balance;
+
+	/// Adds a vesting schedule to a given account.
+	///
+	/// If there already exists a vesting schedule for the given account, an `Err` is returned
+	/// and nothing is updated.
+	fn add_vesting_schedule(
+		who: &AccountId,
+		locked: Self::Balance,
+		per_block: Self::Balance,
+		starting_block: Self::Moment,
+	) -> result::Result<(), &'static str>;
+
+	/// Remove a vesting schedule for a given account.
+	fn remove_vesting_schedule(who: &AccountId);
+}
+
 bitmask! {
 	/// Reasons for moving funds out of an account.
 	#[derive(Encode, Decode)]