sp-std -> core (#3199)

First in a series of PRs that reduces our use of sp-std with a view to
deprecating it.

This is just looking at /substrate and moving some of the references
from `sp-std` to `core`.
These particular changes should be uncontroversial.

Where macros are used `::core` should be used to remove any ambiguity.

part of https://github.com/paritytech/polkadot-sdk/issues/2101

substrate/frame/bounties/src/migrations/v4.rs

@@ -15,6 +15,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use core::str;
 use frame_support::{
 	storage::{generator::StorageValue, StoragePrefixedMap},
 	traits::{
@@ -25,7 +26,6 @@ use frame_support::{
 };
 use sp_core::hexdisplay::HexDisplay;
 use sp_io::{hashing::twox_128, storage};
-use sp_std::str;
 
 use crate as pallet_bounties;