Provide alignment-changing realloc on `GlobalAlloc`

[CAD97]

This is a short-term potentially stabilizable incremental improvement on the status quo.

Zulip discussion

There's at least two benefits to doing this:

• Defining a realloc that takes Layout will allow it to benefit from Enforce that layout size fits in isize in Layout rust#95295
• Over-aligned Vec<u8> can potentially be converted into Vec<u16> without violating the requirement of dealloc with the correct alignment by a layout-changing realloc

This would probably take the shape of unsafe fn grow(&self, ptr: *mut u8, old_layout: Layout, new_layout: Layout) -> *mut u8 { ... } and shrink for GlobalAlloc along with the plumbing to see that through. These should probably be defaulted to call realloc where possible to avoid pessimization of old GlobalAlloc implementers.

Alternatively to providing the grow/shrink split on GlobalAlloc, some alternative symbol to realloc could be used which still allows both grow and shrink. C-inspired bad idea: realloc_s 😛 but resize might be fine. realloc will still exist (maybe deprecated) to point at resize as the improved alternative.