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 <5588131+kianenigma@users.noreply.github.com>

* 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 <5588131+kianenigma@users.noreply.github.com>

substrate/primitives/arithmetic/src/traits.rs

@@ -21,7 +21,7 @@ use codec::HasCompact;
 pub use integer_sqrt::IntegerSquareRoot;
 pub use num_traits::{
 	Zero, One, Bounded, CheckedAdd, CheckedSub, CheckedMul, CheckedDiv,
-	CheckedShl, CheckedShr
+	CheckedShl, CheckedShr, checked_pow
 };
 use sp_std::ops::{
 	Add, Sub, Mul, Div, Rem, AddAssign, SubAssign, MulAssign, DivAssign,
@@ -104,29 +104,41 @@ impl<T: Bounded + Sized, S: TryInto<T> + Sized> UniqueSaturatedInto<T> for S {
 	}
 }
 
-/// Simple trait to use checked mul and max value to give a saturated mul operation over
-/// supported types.
+/// Saturating arithmetic operations, returning maximum or minimum values instead of overflowing.
 pub trait Saturating {
-	/// Saturated addition - if the product can't fit in the type then just use max-value.
-	fn saturating_add(self, o: Self) -> Self;
+	/// Saturating addition. Compute `self + rhs`, saturating at the numeric bounds instead of
+	/// overflowing.
+	fn saturating_add(self, rhs: Self) -> Self;
 
-	/// Saturated subtraction - if the product can't fit in the type then just use max-value.
-	fn saturating_sub(self, o: Self) -> Self;
+	/// Saturating subtraction. Compute `self - rhs`, saturating at the numeric bounds instead of
+	/// overflowing.
+	fn saturating_sub(self, rhs: Self) -> Self;
 
-	/// Saturated multiply - if the product can't fit in the type then just use max-value.
-	fn saturating_mul(self, o: Self) -> Self;
+	/// Saturating multiply. Compute `self * rhs`, saturating at the numeric bounds instead of
+	/// overflowing.
+	fn saturating_mul(self, rhs: Self) -> Self;
+	
+	/// Saturating exponentiation. Compute `self.pow(exp)`, saturating at the numeric bounds
+	/// instead of overflowing.
+	fn saturating_pow(self, exp: usize) -> Self;
 }
 
-impl<T: CheckedMul + Bounded + num_traits::Saturating> Saturating for T {
+impl<T: Clone + One + CheckedMul + Bounded + num_traits::Saturating> Saturating for T {
 	fn saturating_add(self, o: Self) -> Self {
 		<Self as num_traits::Saturating>::saturating_add(self, o)
 	}
+
 	fn saturating_sub(self, o: Self) -> Self {
 		<Self as num_traits::Saturating>::saturating_sub(self, o)
 	}
+
 	fn saturating_mul(self, o: Self) -> Self {
 		self.checked_mul(&o).unwrap_or_else(Bounded::max_value)
 	}
+
+	fn saturating_pow(self, exp: usize) -> Self {
+		checked_pow(self, exp).unwrap_or_else(Bounded::max_value)
+	}
 }
 
 /// Convenience type to work around the highly unergonomic syntax needed