Turn resource adaptors into alias templates

[miscco]

Currently all resource adaptors are templated on their upstream resource.

Transitioning to resource_ref this is superfluous, as we do not need to know the upstream type anymore.

However, we cannot simply remove the template argument, as that would break user code. So we need to first introduce an indirection using an alias:

template<class>
struct resource_adaptor_impl{};

template<class Upstream>
[[deprecated("Use resource_adaptor_ref instead")]] using resource_adaptor = resource_adaptor_impl<Upstream>;

using resource_adaptor_ref = resource_adaptor_impl<int>;

[harrism]

I'm confused by the ref here, because I don't think this is intended to be a reference. Am I correct? Is the idea to make the alias match the name we intend the caller to eventually use?

[miscco]

The issue is how to transition something that is a template to something that is not. I am all ears on how to switch names

[harrism]

I don't have any ideas but answers to my questions might give me some. :) I still have the same questions I wrote above.