Remove constraints on class template proxy and optimize memory layout for small VTABLE

[mingxwa]

When a VTABLE only contains 2 function pointers or less, it is not necessary to dereference through a pointer. This change allows a VTABLE to be embedded into a proxy if small enough. The constraints on class template proxy (basic_facade) has been removed since this optimization needs all the dispatches to be complete types before proxy is instantiated, while proxy<F> is allowed to be incomplete in the definition of a dispatch.

This change also improves proxiable and corresponding helper macros. When the return type of an instantiated operator() of a dispatch D does not match the return type defined in the specific overload of D::overload_types, proxiable used to produce a compile error. The behavior has become SFINAE-safe after this change.

Resolves #69
Resolves #65