Add ensure-ops family methods (#12967)

* add ensure-ops family methods

* fix cargo doc

* add EnsureOp and EnsureOpAssign meta traits

* move ensure module and ArithmeticError to sp-arithmetic

* fix doc examples

* reexport ensure module content

* ensure mod private

* reexport to sp-runtime

* fix doc example

* remove into(). in doc examples, minor doc changes

* remove return value from assign methods

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/arithmetic/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* cargo fmt

* Apply suggestions from code review

* ".git/.scripts/fmt.sh" 1

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: command-bot <>