ChainSpec extensions (#3692)

* Add some chainspec tests and make sure we validate it.

* Manual implementation of Extension + Forks definitions.

* Move chain spec to separate crate.

* Allow using ChainSpec with extensions.

* Renames.

* Implement Extension derive.

* Implement Extension for Forks.

* Support specifying fork blocks.

* make for_blocks work

* Support forks correctly.

* Add a bunch of docs.

* Make fork blocks optional.

* Add missing docs.

* Fix build.

* Use struct for check_block params.

* Fix tests?

* Clean up.

substrate/core/test-client/src/lib.rs

@@ -187,6 +187,7 @@ impl<Executor, Backend, G: GenesisInit> TestClientBuilder<Executor, Backend, G>
 			self.backend.clone(),
 			executor,
 			storage,
+			Default::default(),
 			self.execution_strategies,
 		).expect("Creates new client");