Auto merge of #55366 - Amanieu:stable_layout, r=Amanieu

Auto merge of #55366 - Amanieu:stable_layout, r=Amanieu

Add tracking issue for Layout methods (and some API changes)

These methods are already useful when used with the stable global allocator API (stabilized in #51241).

```rust
pub fn align_to(&self, align: usize) -> Result<Layout, LayoutErr>;
pub fn padding_needed_for(&self, align: usize) -> usize;
pub fn repeat(&self, n: usize) -> Result<(Layout, usize), LayoutErr>;
pub fn extend(&self, next: Layout) -> Result<(Layout, usize), LayoutErr>;
pub fn repeat_packed(&self, n: usize) -> Result<Layout, LayoutErr>;
pub fn extend_packed(&self, next: Layout) -> Result<Layout, LayoutErr>;
pub fn array<T>(n: usize) -> Result<Layout, LayoutErr>;
```

cc #32838

r? @SimonSapin