Staking module: approximate fraction into perbill to avoid overflow (#2889)

* approximate fraction into perbill

* test

* fix comment

* line width

* bump impl version

* rename test for better naming

* test overflow

* Apply suggestions from code review

Co-Authored-By: Niklas Adolfsson <niklasadolfsson1@gmail.com>

substrate/node/runtime/src/lib.rs

@@ -59,7 +59,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 97,
-	impl_version: 98,
+	impl_version: 99,
 	apis: RUNTIME_API_VERSIONS,
 };