cbor: drop std feature, use core in tests (#327)

libraries/cbor/src/lib.rs

@@ -12,11 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![cfg_attr(not(feature = "std"), no_std)]
+#![no_std]
 
 extern crate alloc;
-#[cfg(feature = "std")]
-extern crate core;
 
 pub mod macros;
 pub mod reader;