feat(arena2): implement dynamic alignment for ArenaAllocator

[shruti2522]

Replaced hardcoded arena alignment with dynamic type based alignment with min_alignment configuration (arena2)

[nekevss]

issue: I think this should really just be a const parameter vs. a regular parameter.

Had you looked at using const parameters for this?

[shruti2522]

I looked into this, here's what I have found so far. One possible design would be to make MIN_ALIGN a const parameter on ArenaAllocator and compute max(MIN_ALIGN, ALIGN) when creating a new arena. Since both values would be const, the compiler could resolve max at compile time.

But I think this doesn’t really help here since arena_size is still a runtime value and Arena::try_init uses Layout::from_size_align(arena_size, alignment). Because the size is runtime, the layout cannot be fully computed at compile time anyway.

It would also mean many parts of the code would also need MIN_ALIGN as a generic parameter but that would involve a lot of changes without improving runtime performance. Also, instead of ArenaAllocator<'alloc>, we would need types like ArenaAllocator<'alloc, 8>, and allocators with different alignments would become incompatible types.

So I think the current design keeps the code simpler