Feature const_alloc_layout makes these methods on Layout const:

• padding_needed_for
• for_value
• for_value_raw
• pad_to_align
• array

Making more methods of alloc::Layout const allows computing alignment/size information for arbitrary (sized) types at compile-time. While mem::size_of and mem::align_of are already const and Layout is solely based on those, there is no guarantee that a const derived from these functions will be exactly the same as what is used in a call to alloc::alloc. Constifying Layout makes this possible.

PR #67494, related to #67520