Resolve review comments of P3086R1 from the ISO C++ committee

[mingxwa]

As per review comments of P3086R1, two design issues were raised. More details of the review are documented here. The two issues are:

1. the semantics of concept facade should expect tuple-like types rather than specializations of std::tuple to stay consistency with the existing standard.
2. make_proxy lacks allocator support. Since proxy does not rely on this facility, splitting into a separate paper is recommended.

For 1, the implementation of was updated in this change to accept any tuple-like types in concept facade, PRO_DEF_FACADE and proxy. Another case was added in tests/proxy_traits_tests.cpp.

For 2, a new API allocate_proxy was added in this change. 15 new cases were added in tests/proxy_creation_tests.cpp.

Closes #74
Closes #75