Optimize upb_Arena memory allocation for repeated growths

[copybara-service]

Optimize upb_Arena memory allocation for repeated growths

Introduce a memory pool within upb_Arena to recycle power-of-2 sized blocks (on 32-bit) and map-entry-aligned sizes (on 64-bit). This addresses the "arena leak" pattern where structures like repeated fields (arrays) and maps (hash tables) reallocate repeatedly and leave abandoned, unreclaimable blocks in the arena.

Key changes:

• Implement upb_Arena_AllocPool and upb_Arena_FreePool in arena.c using an in-place sorted linked list of size classes (LIFO stacks of free blocks), avoiding any intermediate allocations.
• Use dynamic size classes of the form UPB_MAP_ENTRY_SIZE * 2^k, which dynamically resolves to 24 * 2^k on 64-bit and 16 * 2^k on 32-bit (collapsing to pure powers of 2).
• Achieve 0% memory overhead for all maps >= 48 bytes by matching the map resize growth system exactly.
• Integrate the pool into upb_Array reallocation with Capacity Coalescing, allowing arrays to safely use these same size classes and share memory blocks with maps.
• Fix a critical memory corruption bug by protecting the array's inline initial data block from being freed to the pool.
• Integrate the pool into upb_table (strtable, exttable, inttable) resizes to reclaim old entry arrays.
• Guard the implementation with extensive UPB_ASSERTs for alignment, sizes, sorting, and stack invariants.
• Ensure full compatibility and safety under ASAN/HWASAN (handling tag-stripping and aligned poisoning).