Budge

- Add `Transaction.cleared` boolean field.
- Add `RepeatingTransaction.last_cleared` date field. All `Transactions` yielded
will have their `cleared` field set to true if the date is before the value of
`last_cleared`. The same rule applies to `RepeatingTransfer`.
- Add `cleared: bool | None` parameters to `Account` methods, where a non-`None`
value filters account transactions by the given value for `cleared`.

- Removed the ability to set a transaction's amount to a callback until I've
redesigned how that functionality should work.

- Added `Account.running_balance` that iterates through each transaction in an
account for the given date range, and includes the current running balance of
the account.

- Accept any value that can be passed to the constructor of `stockholm.Money`
wherever a `Money` object was required as an argument.
- Accept a function that accepts the object instance as an argument and returns
a `Money`-convertible value as an `amount` parameter.
- Swapped positions of the `amount` and `description` fields on `Transaction`.

- Set account property on `Transaction` objects generated from
`RepeatingTransaction` objects.
- Allow `dateutil.rrule.rruleset` in `RepeatingTransaction.schedule` and
`RepeatingTransfer.schedule`.
- Refactor internal storage of `Collection` instances to use `dict`.
- Exclude `Transaction` objects generated from a `RepeatingTransaction` when
iterating over an account's transactions, if a manually entered `Transaction`
matches the generated `Transaction`.

- Refactored `Account.transactions` and `Account.repeating_transactions` to a
`Collection` class that assigns parent references on child items.
- Refactored generators to prefer `yield from` over returning a generator.