Refactor sr-api to not depend on client anymore (#4086)

* Refactor sr-api to not depend on client anymore

* Fix benches

* Apply suggestions from code review

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Apply suggestions from code review

substrate/core/authority-discovery/primitives/src/lib.rs

@@ -18,7 +18,6 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
-use client::decl_runtime_apis;
 use rstd::vec::Vec;
 use sr_primitives::RuntimeDebug;
 
@@ -29,7 +28,7 @@ pub struct Signature(pub Vec<u8>);
 #[cfg_attr(feature = "std", derive(Hash))]
 pub struct AuthorityId(pub Vec<u8>);
 
-decl_runtime_apis! {
+sr_api::decl_runtime_apis! {
 	/// The authority discovery api.
 	///
 	/// This api is used by the `core/authority-discovery` module to retrieve our