Generic Normalize impl for arithmetic and npos-elections (#6374)

* add normalize

* better api for normalize

* Some grumbles

* Update primitives/arithmetic/src/lib.rs

Co-authored-by: Guillaume Thiolliere <[email protected]>

* More great review grumbles

* Way better doc for everything.

* Some improvement

* Update primitives/arithmetic/src/lib.rs

Co-authored-by: Bernhard Schuster <[email protected]>

Co-authored-by: Guillaume Thiolliere <[email protected]>
Co-authored-by: Bernhard Schuster <[email protected]>