Make all PerThing types implement all trait methods on the type (#5422)

This removes the requirement to import `PerThing` to use a type that
implements the trait.

substrate/primitives/arithmetic/fuzzer/src/per_thing_rational.rs

@@ -24,8 +24,7 @@
 
 use honggfuzz::fuzz;
 use sp_arithmetic::{
-	PerThing, PerU16, Percent, Perbill, Perquintill, assert_eq_error_rate,
-	traits::SaturatedConversion,
+	PerThing, PerU16, Percent, Perbill, Perquintill, traits::SaturatedConversion,
 };
 
 fn main() {