Extend PerThing + Saturating (#5281)

* Extend PerThing + Saturating

* Add saturating_pow to Saturating
* Add saturating_truncating_mul to PerThing (rounding-down mul)
* Add saturating_reciprocal_mul to PerThing (divide x by perthing)
* Provide default methods where possible

* Restore const functions

* Fix test

* Update primitives/arithmetic/src/per_things.rs

Co-Authored-By: Kian Paimani <[email protected]>

* Add comment and test verifying no overflow

* Formatting

* Fix possible overflow and change type constraint

* Use overflow pruning for all mul

* Formatting and comments

* Improve comments and names

* Comments in `rational_mul_correction` explain overflow aversion.

* Test rational_mul_correction

* Formatting

* Docs and formatting

* Add new trait methods to Perthing type impl

* Fix signature

* saturating_pow for Delegations

* Add missing trait method to impl

Co-authored-by: Kian Paimani <[email protected]>