new crate sc-light (#6235)

* sc-light

* remove unused deps

* fix line width

* move more fns to sc_light

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

@@ -46,7 +46,7 @@ use sp_runtime::traits::{Block as BlockT, BlakeTwo256};
 use sc_service::client::{LocalCallExecutor, ClientConfig};
 
 /// Test client light database backend.
-pub type LightBackend<Block> = client::light::backend::Backend<
+pub type LightBackend<Block> = sc_light::Backend<
 	sc_client_db::light::LightStorage<Block>,
 	BlakeTwo256,
 >;