Companion for sub/8176 (#2622)

* Merge

* Fixes

* Fix build

* remove dep.

* undo dep.

* upadte substrate

* cargo run --release --features=runtime-benchmarks -- benchmark --chain=polkadot-dev --steps=50 --repeat=20 --pallet=pallet_staking --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --header=./file_header.txt --output=./runtime/polkadot/src/weights/

* Fix lock

* revert lock; cargo update -p sp-io

* from_rational_approx -> from_rational

* Silence more warnings

Co-authored-by: Gav Wood <[email protected]>
Co-authored-by: Shawn Tabrizi <[email protected]>
Co-authored-by: Parity Benchmarking Bot <[email protected]>