Add an internal ASSERT macro

This macro has some advantages over the standard library one:
 - It prints the place where the macro was triggered
 - It "references" the condition even in Release to avoid warnings
 - In release, if possible, it gives compiler hints

It is basically is stripped down version of the ASSERT macros I wrote
for the Daemon engine in src/common/Assert.h

This commit also removes the stray "backend" namespaces for common/
code.

src/common/Assert.cpp

@@ -0,0 +1,22 @@
+// Copyright 2017 The NXT Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "common/Assert.h"
+
+#include <iostream>
+
+void HandleAssertionFailure(const char* file, const char* function, int line, const char* condition) {
+    std::cerr << "Assertion failure at " << file << ":" << line << " (" << function << "): " << condition << std::endl;
+    NXT_BREAKPOINT();
+}