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

failtest/ref_4.cpp

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