Support for MSVC

[jfalcou]

Last night, I installed MSVC 2019 Preview which has better support for the kind of C++20 we do.
As this is a low frequency task, this discussion is here to monitor my findings.

[jfalcou]

05/13/2021 - First try at compiling the test suite

Trivial stuff:

• issue with short from concept parameters: f(concept auto x) not being supported
• issue with if constexpr( requires { complex crap; } ) that requires adding a proper concept
• Some massaging/simplification of one liner TMP stint is required in a few spo

Those trivial fixes make unit.arch and unit.meta to compiles with no hassle and at a decent speed.

Now, unit.internals fails cause mSVC thinks detail::make is ambiguous.
The problem is the error is just that "call is ambiguous".

I suspect I need to physically differentiate make(T) et make(T,Ts) into make(T) and make(T,U,Ts).