Improve facade infrastructure

[mingxwa]

Changes

• Split overload_types from "Dispatch" and added new named requirements of "Convention". The tables below summarizes each dimension of "Facade":

Name	Description	Fields
Facade	Facade of runtime polymorphism	convention_types: tuple-like collection of Convention reflection_types : tuple-like collection of Reflection constraints: pro::proxiable_ptr_constraints
Convention	Runtime calling convention	dispatch_type: Dispatch overload_types: tuple-like collection of overload signatures
Dispatch	Callable type for a convention	Default noexcept constructor: Creation of a Dispatch, encouraged to be trivial operator() of arbitrary type T& and arguments: Invoke with lvalue reference of type T [Optional] operator() of arbitrary type const P& and arguments: Invoke with the underlying pointer type [Optional] operator() of std::nullptr_t and arguments: Default invocation behavior accessor<P>: Accessor of a proxy type
Reflection	Compile-time reflection type that can be inspected at runtime	constexpr constructor of type std::in_place_type_t<P>, where P is an underlying pointer type
Accessor	Accessor of proxy that can inject more APIs to improve syntax	Default noexcept constructor: Creation of an Accessor, encouraged to be trivial

• Added support for multiple reflection types.
• Renamed PRO_DEF_MEMBER_DISPATCH into PRO_DEF_MEM_DISPATCH (and the _WITH_DEFAULT version) and removed overload types from the macro.
• Removed overload types from macros PRO_DEF_FREE_DISPATCH (and the _WITH_DEFAULT version).
• Replaced macros PRO_MAKE_DISPATCH_PACK(), PRO_DEF_FACADE and the 3 instantiations of pro::proxiable_ptr_constraints with a builder type pro::facade_builder implemented with pure template metaprogramming. The new syntax to define a facade is as follows:

PRO_DEF_MEM_DISPATCH(MemDraw, Draw);
PRO_DEF_MEM_DISPATCH(MemArea, Area);

struct Copyable : pro::facade_builder
    ::support_copy<pro::constraint_level::nontrivial>
    ::build {};

struct Drawable : pro::facade_builder
    ::add_convention<MemDraw, void(std::ostream&)>
    ::add_convention<MemArea, double() noexcept>
    ::add_facade<Copyable>
    ::build {};

• Revised implementation of pro::details::dependent_t and exposed as pro::lazy_eval_t and pro::lazy_eval() to facilitate definition of a custom dispatch.
• For dispatch built from free functions, APIs are exposed as friend functions rather than member functions.
• Revised implementation of unit tests.