…metadata within `Arena` chunk (#21869)

Large change to how raw transfer stores metadata about the allocations it uses.

Previously the two metadata structures `RawTransferMetadata` and `FixedSizeAllocator` after the chunk's `ChunkFooter`.

This had a few problems:

1. It was confusing and unwieldy - a recipe for bugs.
2. `ChunkFooter`'s memory was exposed on JS side - a bit unsafe as altering bytes in this region could easily trigger UB.
3. It entwined `Arena` (which is just the thing that allocates) with the details of exactly _what_ raw transfer allocates.
4. Imposed annoying alignment requirements, because `ChunkFooter` must be aligned on 16, and so anything after it must ensure it doesn't break that invariant.

Old layout:

```
                                                        WHOLE BLOCK - aligned on 4 GiB
<-----------------------------------------------------> Allocated block (`BLOCK_SIZE` bytes)

                                                        ALLOCATOR
<----------------------------------------->             `Allocator` chunk (`CHUNK_SIZE` bytes)
                                     <---->             `ChunkFooter` (aligned on 16)
<----------------------------------->                   `Allocator` chunk data storage (for AST)
                                                        (`ACTIVE_SIZE` bytes)

                                                        METADATA
                                           <---->       `RawTransferMetadata`
                                                 <----> `FixedSizeAllocatorMetadata`

                                                        BUFFER SENT TO JS
<----------------------------------------------->       Buffer sent to JS (`BUFFER_SIZE` bytes)
```

This PR moves `RawTransferMetadata` and `FixedSizeAllocatorMetadata` into the chunk itself. New layout:

```
                                                        WHOLE BLOCK - size 2 GiB - 16, aligned on 4 GiB
<-----------------------------------------------------> Allocated block (`BLOCK_SIZE` bytes)

                                                        ARENA
<-----------------------------------------------------> Chunk (fills whole block)
<-------------------------------------->                Allocatable region for AST (`ACTIVE_SIZE` bytes)
                                        <--->           `RawTransferMetadata`
                                             <--->      `FixedSizeAllocatorMetadata`
                                                  <---> `ChunkFooter` (aligned on 16, last in block)

                                                        BUFFER SENT TO JS
<------------------------------------------->           Buffer sent to JS (`BUFFER_SIZE` bytes)
```

`FixedSizeAllocatorMetadata` and `ChunkFooter` are no longer in the region which is shared with JS side. As far as `Arena` is concerned, they're now just some data (like any other data) which is allocated in the arena.

Also:

- Introduce more consistency to the naming of constants which specify the size and position of these various data structures in the arena.
- Add more const assertions to ensure everything is laid out and aligned as it should be.