Add macros to simplify syntax of creating dispatches and facades

[mingxwa]

Changes

• Added macros DEFINE_MEMBER_DISPATCH, DEFINE_FREE_DISPATCH, DEFINE_FACADE, and DEFINE_COPYABLE_FACADE. These macros are not intended to be included in the proposal of standardization, but should make the library easier to use before the static reflection standards are stabilized.
• Removed class template pro::dispatch_adapter since it is shadowed by the macros.
• Fixed a bug: When the required return type of an overload is void, but the concrete return type of a dispatch is not void, the code won't compile.
• Updated unit tests and sample code accordingly.

Quick glance

Before:

struct Draw : pro::dispatch<void(std::ostream&)> {
  void operator()(const auto& self, std::ostream& out) { self.Draw(out); }
};
struct Area : pro::dispatch<double()> {
  double operator()(const auto& self) { return self.Area(); }
};
struct DrawableFacade : pro::facade<Draw, Area> {};

After:

DEFINE_MEMBER_DISPATCH(Draw, Draw, void(std::ostream&));
DEFINE_MEMBER_DISPATCH(Area, Area, double());
DEFINE_FACADE(DrawableFacade, Draw, Area);

[suy]

One nitpick: in places like DEFINE_MEMBER_DISPATCH(Area, Area, double()); it is not very clear what each "Area" is. One is the name of the struct, the other the member function called, but which is which? This is more obvious in the DEFINE_FREE_DISPATCH(ToString, to_string, std::string()); example. Naming is hard. :-)

[mingxwa]

One nitpick: in places like DEFINE_MEMBER_DISPATCH(Area, Area, double()); it is not very clear what each "Area" is. One is the name of the struct, the other the member function called, but which is which? This is more obvious in the DEFINE_FREE_DISPATCH(ToString, to_string, std::string()); example. Naming is hard. :-)

That is true. I tried to refactor everything required by proxy into a namespace poly in the sample code to let people aware each macro defines a name. Does it look better now?

[suy]

It helps, thank you!