safe multi-era slashing for NPoS (#3846)

* define slashing spans

* tests and pruning for slashing-spans record

* validators get slashed before nominators

* apply slash to nominators as well

* chill and end slashing spans

* actually perform slashes

* integration (tests failing)

* prune metadata

* fix compilation

* some tests for slashing and metadata garbage collection

* correctly pass session index to slash handler

* test span-max property for nominators and validators

* test that slashes are summed correctly

* reward value computation

* implement rewarding

* add comment about rewards

* do not adjust slash fraction in offences module

* fix offences tests

* remove unused new_offenders field

* update runtime version

* fix up some docs

* fix some CI failures

* remove no-std incompatible vec! invocation

* try to fix span-max rounding error

* Update srml/staking/src/slashing.rs

Fix type: winow -> window

Co-Authored-By: Tomasz Drwięg...