Big 853: replace enable_if in Ref<> ctor by static assertions and add failtests for Ref<>

failtest/ref_2.cpp

@@ -0,0 +1,15 @@
+#include "../Eigen/Core"
+
+using namespace Eigen;
+
+void call_ref(Ref<VectorXf> a) { }
+
+int main()
+{
+  MatrixXf A(10,10);
+#ifdef EIGEN_SHOULD_FAIL_TO_BUILD
+  call_ref(A.row(3));
+#else
+  call_ref(A.col(3));
+#endif
+}