Decouple Phragmen from Staking. (#3498)

* Move phragmen to primitives

* Improved docs

* New crate.

* Update lock.

* Fix dependency.

* Fix build.

* Add basic testing and truth-value implementation with float types

* Update srml/staking/src/lib.rs

* Nits.

* Bump.

* Fix benchmarks.

substrate/node/runtime/src/lib.rs

@@ -80,7 +80,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 154,
-	impl_version: 155,
+	impl_version: 156,
 	apis: RUNTIME_API_VERSIONS,
 };