Rename StorageMap::exists to ::contains_key (Resolves #4839) (#4847)

* rename StorageMap::exists(key) to ::contains_key(key)

* bump impl_version

substrate/bin/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: 214,
-	impl_version: 1,
+	impl_version: 2,
 	apis: RUNTIME_API_VERSIONS,
 };