Improve natvis

[K-ballo]

• extend support from 10 to 40 alternatives,
• split single-buffer vs double-buffer definitions,
• adjust display string to follow MS definition for std::variant

[pdimov]

@vinniefalco Any objections to this?

[vinniefalco]

Looks better than my version, so if this works I would give it a three thumbs up.

[vinniefalco]

@K-ballo did you test this?

[K-ballo]

@K-ballo did you test this?

Here are some examples:

variant<bool, int, double> v0{ 0.0 }; // single storage
variant<bool, int, double, int, int, int, int, int, int, int> v1{ 1.0 }; // 10 elements storage
variant<evil, bool, int, double> v2{ 2.0 }; // double storage
variant<evil, bool, int, double> v3; // double storage
v3.emplace<double>(3.0); // use second storage