Replace macro PRO_DEF_WEAK_DISPATCH with class template weak_dispatch

[mingxwa]

This change replaced macro PRO_DEF_WEAK_DISPATCH with class template weak_dispatch<D>. Specifically,

• Added a new exception type not_implemented, inspired by NotImplementedException in .NET. However, not_implemented is not derived from std::logic_error or std::runtime_error due to freestanding limitations.
• weak_dispatch<D> will add a default implementation to an existing dispatch type D. The default implementation simply throws not_implemented.
• Updated unit tests accordingly.
• Added [[deprecated]] attribute to types defined with PRO_DEF_WEAK_DISPATCH.
• Added error message to bad_proxy_cast.
• Documentation and sample code will be updated later.

Note that the return type of weak_dispatch's default implementation is details::wildcard, which can syntactically be convertible to any type, while it will never be constructed.