Simplify the semantics of facade per spec

[mingxwa]

To simplify the semantics of proxy, several changes has been made as per the latest spec:

1. Defining repeated dispatches in a facade is no longer supported. An author of a facade should guarantee every dispatch is unique. Helper macros will serve this purpose with metaprogramming at compile time.
2. concept basic_facade was strengthened with the following additions: a) F::dispatch_types shall be a specialization of std::tuple; b) The type of F::constraints shall be exactly const pro::proxiable_ptr_constraints, no implicit conversion is allowed; c) the maximum size and alignment defined by F::constraints shall follow the rule of C++; d) When F::reflection_type is not void, it shall be trivially copyable.
3. concept facade was strengthen with the following addition: Each dispatch type defined by F::dispatch_types shall be trivially default constructible.

Some test cases are added to verify the constraints work as expected. No functional changes.