add rvalue reference support for static_variant

[huangminghuang]

This PR allows one to use static_variant as rvalue reference with visitor as follows:

  static_variant<string, vector<char>> value =...;
  std::move(value).visit( overloaded {
    [](string&& v) { ...},
    [](vector<char>&& v) {...}
  } };

[heifner]

Can you add a tests for this to test_static_variant.cpp