Remove `As` (#2602)

* Start to remove the `As` bound on `SimpleArtithmetic`

This just introduces standard numeric bounds, assuming a minimum of
`u32`. Also included is a saturating from/into trait allowing ergonomic
infallible conversion when you don't care if it saturates.

* Remove As from Balances trait

* Remove As from Aura module

* Remove As from Babe module

* Expunge `As` from contract

* Council module

* Democracy

* Finality tracker

* Grandpa

* First bit of indices

* indices

* Line lengths

* session

* system

* Staking

* Square up all other uses of As.

* RHD update

* Fix build/test

* Remove As trait

* line widths

* Remove final As ref

* Update srml/staking/src/lib.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Update core/client/src/cht.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Update core/client/db/src/light.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>

* whitespace

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>
Co-Authored-By: André Silva <[email protected]>

* Bring back u32 check for number on CLI