Fix imports and features for core

Author: workingjubilee

crates/core_simd/src/lib.rs

@@ -1,7 +1,6 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![feature(
     const_fn_trait_bound,
-    const_panic,
     platform_intrinsics,
     repr_simd,
     simd_ffi,

crates/core_simd/src/swizzle.rs

@@ -1,5 +1,5 @@
 use crate::simd::intrinsics;
-use crate::{LaneCount, Simd, SimdElement, SupportedLaneCount};
+use crate::simd::{LaneCount, Simd, SimdElement, SupportedLaneCount};
 
 /// Constructs a new vector by selecting values from the lanes of the source vector or vectors to use.
 ///