Rollup merge of #78305 - ChrisDenton:const-layout, r=oli-obk

Rollup merge of #78305 - ChrisDenton:const-layout, r=oli-obk

Stabilize alloc::Layout const functions

Stabilizes #67521. In particular the following stable methods are stabilized as `const fn`:

* `size`
* `align`
* `from_size_align`

Stabilizing `size` and `align` should not be controversial as they are simple (usize and NonZeroUsize) fields and I don't think there's any reason to make them not const compatible in the future. That being true, the other methods are trivially `const`. The only other issue being returning a `Result` from a `const fn` but this has been made more usable by recent stabilizations.