C++17 structured bindings

[guwirth]

Support C++17 Structured Bindings.

Sample:

int[2] f();
auto [ x, y ] = f();

tuple<T1,T2,T3> g();
auto [a,b,c] = g();

struct MyStruct {
  int x;
  double y;
};

MyStruct h();
auto [u,v] = h();

std::map<string, double> mymap;
...
for (const auto& [key,val] : mymap) {
  std::cout << key << ": " << val << std::endl;
}

[guwirth]

related to #1104