[FR] Add std::function_ref equivalent functionality

[psiha]

A 'proxy_ref': so that we don't pass around proxy const & (thereby incurring a double indirection).

[mingxwa]

If I understand correctly, proxy already provides the capability. You can simply instantiate a proxy<F> with any raw pointer type P (although proxy<F> still has nontrivial move ctor and dtor, it behaves like a raw pointer).

If proxy<F> is intended not to take any ownership for all P, it is allowed to fine tune performance like std::function_ref does. Lifetime semantics is associated with the definition of a facade (F::constraints). If you are using PRO_DEF_FACADE to define a facade, the constraints can be specified on the 3rd parameter, e.g.,

PRO_DEF_MEMBER_DISPATCH(Draw, void(std::ostream& out));
PRO_DEF_FACADE(DrawableRef, Draw, pro::trivial_ptr_constraints);

proxy<DrawableRef> will have trivial copy ctor and move ctor, but it can no longer hold any smart pointers (e.g., std::unique_ptr std::shared_ptr or pro::details::sbo_ptr).

[psiha]

That's not the same: a function_ref-like thingy does not need an SBO (or heap allocation) for any kind of target object - as it references it directly (as opposed to owning it) - it can/should be able to bind to a temprary (function argument).
A reference to a trivial proxy<> has two indirections: reference to the proxy and then a reference to the vtable (and possibly an additional one to the target if it did not fit into the SBO storage).
trivial_ptr_constraints will make proxy the size of two pointers, like function_ref but will require a heap allocation and a nontrivial destructor for larger-than-a-pointer targets or not support those at all.

[mingxwa]

trivial_ptr_constraints will make proxy the size of two pointers, like function_ref but will require a heap allocation and a nontrivial destructor for larger-than-a-pointer targets or not support those at all.

In the design of proxy, if trivial_ptr_constraints is specified in a facade definition F, proxy<F> is a trivial type. It is covered in our unit tests.

[mingxwa]

The dereferencing of small vtable is triaged as a QoI issue (filed #69).