Fix (stacked-borrows-)UB found by Miri

src/byteset/scalar.rs

@@ -67,7 +67,7 @@ pub fn inv_memrchr(n1: u8, haystack: &[u8]) -> Option<usize> {
     let loop_size = cmp::min(LOOP_SIZE, haystack.len());
     let align = USIZE_BYTES - 1;
     let start_ptr = haystack.as_ptr();
-    let end_ptr = haystack[haystack.len()..].as_ptr();
+    let end_ptr = unsafe { haystack.as_ptr().add(haystack.len()) };
     let mut ptr = end_ptr;
 
     unsafe {

src/ext_slice.rs

@@ -3063,11 +3063,8 @@ pub trait ByteSlice: Sealed {
         // Finally, we are only dealing with u8 data, which is Copy, which
         // means we can copy without worrying about ownership/destructors.
         unsafe {
-            ptr::copy(
-                self.as_bytes().get_unchecked(src_start),
-                self.as_bytes_mut().get_unchecked_mut(dest),
-                count,
-            );
+            let ptr = self.as_bytes_mut().as_mut_ptr();
+            ptr::copy(ptr.add(src_start), ptr.add(dest), count);
         }
     }
 }