Fix undefined behavior in swap with the miri stacked borrows model.

The `swap` function passed with `MIRIFLAGS=-zmiri-tree-borrows`
But failed when ran through the stacked borrows model, because
`get_unchecked_mut` returns `&mut` references which we need two of.

by using `as_mut_ptr()` we can avoid creating multiple mutable references
from a single one satisfying uniqueness.

src/core/option.rs

@@ -225,8 +225,9 @@ impl<T> Core<T> for OptionCore<T> {
         // We can't just have two mutable references, so we use `ptr::swap`
         // instead of `mem::swap`. We do not use the slice's `swap` method as
         // that performs bound checks.
-        let pa: *mut _ = self.data.get_unchecked_mut(a);
-        let pb: *mut _ = self.data.get_unchecked_mut(b);
+        let p = self.data.as_mut_ptr();
+        let pa: *mut _ = p.offset(a as isize);
+        let pb: *mut _ = p.offset(b as isize);
         ptr::swap(pa, pb);
     }
 }