# Description
## What?
Make it possible for other pallets to implement their own logic when a
slash on a balance occurs.

## Why?
In the [introduction of
holds](https://github.com/paritytech/substrate/pull/12951) @gavofyork
said:
> 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.

At Polimec we needed to find a way to reduce the vesting schedules of
our users after a slash was made, and after talking to @Kianenigma at
the Web3Summit, we realized there was no easy way to implement this with
the current traits, so we came up with this solution.



## How?
- First we abstract the `done_slash` function of holds::Balanced to it's
own trait that any pallet can implement.
- Then we add a config type in pallet-balances that accepts a callback
tu...