Rewrite proportional slashing to to not leak dust into chunks that should not be slashed.

[kianenigma]

The purpose of proportional slashing, as per #10982 and #11782 to slash certain chunks and the active ledger, and leave the rest untouched.

This helps applications like nomination pools on top of the current staking provide better guarantees to their users.

For example, in Ledger { active: 10, unlokcing: vec![(10, 10) (10, 11)] }, we want to slash 5 from active and 5 from first unlocking chunk, instead of slashing 10 out of active.

The current implementation does achieve this, but since all slashes are rounded down, it is very likely that the sum of the proportional slashes are not enough to cover the slash, and some very small dust amount is slashed from the "first chunk that should have not been affected".

Examples of this can be found in proportional_ledger_slash_works tests.

This is not a big deal, but fixing it will clean up both the code and the interface a bit. All changes must be contained in fn slash in impl Ledger.

[NingLin-P]

Just found that a little change can fix this issue, namely changing slash ratio to remaining ratio, PTAL #12058