Add `async::pmr_context`

[kammce]

This is a simple context that accepts a memory resource as an input and uses it to allocate the memory for the stack. It also calls cancel() deallocates the memory on destruction.

[kammce]

I just realized today that this object has a bit of a problem. async::context isn't movable and if this inherits it, then its also not going to be movable. But having a version of a context that is movable seems very useful. We can consider this like a smart pointer where you need to use the operator*() and operator->() to access the context. That way, it just holds a pointer to the object and a pointer to the memory resource it used to allocate the memory.

So this type will not inherit async::context but instead will have an API async::context& operator*();.