default capacity should be independent of architecture

[giuliano-macedo]

The default capacity of an arena was being set to ARENA_REGION_DEFAULT_CAPACITY * sizeof(uintptr_t) on new_region, therefore I think it makes sense to divide it on arena_alloc so that ARENA_REGION_DEFAULT_CAPACITY would mean the default capacity in bytes, not in terms of N pointers, if the latter was the intent, then I think ARENA_REGION_DEFAULT_CAPACITY should have a different name.