forbid moves out of slices

forbid moves out of slices

The wording of RFC #495 enables moves out of slices. Unfortuantely, non-zeroing
moves out of slices introduce a very annoying complication: as slices can
vary in their length, indexes from the start and end may or may not overlap
depending on the slice's exact length, which prevents assigning a particular
drop flag for each individual element.

For example, in the code

```Rust
fn foo<T>(a: Box<[Box<[T]>]>, c: bool) -> T {
    match (a, c) {
        (box [box [t, ..], ..], true) => t,
        (box [.., box [.., t]], false) => t,
        _ => panic!()
    }
}
```

If the condition is false, we have to drop the first element
of `a`, unless `a` has size 1 in which case we drop all the elements
of it but the last.

If someone comes with a nice way of handling it, we can always re-allow
moves out of slices.

This is a [breaking-change], but it is behind the `slice_patterns` feature
gate and was not allowed until recently.