Instead of returning a raw pointer, `LinearAllocator.construct(...)` now returns
a `destruct_ptr`, which is similar to `unique_ptr`, but does not deallocate
the memory and only calls the destructor instead.
The main change is that large allocations are done separately now.
Also, buffers that small allocations are packed into, have a maximum
size now. Using larger buffers does not really provider performance
benefits, but increases wasted memory.
