Add C++17 structured bindings support for fixed-size Matrix and Array

libeigen/eigen!2336

Closes #2247

failtest/structured_bindings_dynamic_array.cpp

@@ -0,0 +1,16 @@
+#include "../Eigen/Core"
+
+// Reproduces the "Dynamic-sized Array breaks tuple_size" bug: the Array
+// specialization had the same enable_if_t base-clause issue as Matrix. Compile
+// must fail for ArrayXd.
+#ifdef EIGEN_SHOULD_FAIL_TO_BUILD
+#define ROWS Eigen::Dynamic
+#define COLS Eigen::Dynamic
+#else
+#define ROWS 3
+#define COLS 1
+#endif
+
+#include <tuple>
+
+int main() { return static_cast<int>(std::tuple_size<Eigen::Array<double, ROWS, COLS>>::value); }