Make use of NLL in client crate (#1436)

* fix: set edition to 2018 in Cargo.toml.

* fix: refactor function to make use of NLL.

* fix: result of applying 'cargo fix --edition' command.

* fix: removes extern crate

* fix: remove module uses from lib.rs

* fix: tests imports

substrate/core/client/src/runtime_api.rs

@@ -32,10 +32,10 @@ pub use rstd::{slice, mem};
 use rstd::result;
 pub use codec::{Encode, Decode};
 #[cfg(feature = "std")]
-use error;
+use crate::error;
 use rstd::vec::Vec;
 use primitives::OpaqueMetadata;
-
+use sr_api_macros::decl_runtime_apis;
 
 /// Something that can be constructed to a runtime api.
 #[cfg(feature = "std")]