Choose a better type/strategy for "stack memory"

[kammce]

mem::strong_ptr<std::span<byte>> is not an easy type to create. And if even if you do create one, its non-owning, meaning its dangerous to use. An owning replacement would be great.

[kammce]

The solution is to have ownership over the memory and to perform the allocate within the context rather than asking for the stack memory to be passed to us.

And after some long time thinking this over, in order to keep the size of async::context within a cache line, we will add another virtual API to scheduler called get_allocator() which returns a memory resource. That way, we only need a single strong_ptr to the scheduler and it contains access to the memory resource to be used for creating stack memory for contexts. That same allocator will be used to free the memory om context destruction.