Rename Iterator::get_unchecked

It's possible for method resolution to pick this method over a lower
priority stable method,  causing compilation errors. Since this method
is permanently unstable, give it a name that is very unlikely to be used
in user code.

library/alloc/src/vec.rs

@@ -2981,7 +2981,7 @@ impl<T> Iterator for IntoIter<T> {
         self.len()
     }
 
-    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item
+    unsafe fn __iterator_get_unchecked(&mut self, i: usize) -> Self::Item
     where
         Self: TrustedRandomAccess,
     {