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/tests/unittests/MathTests.cpp

@@ -16,8 +16,6 @@
 
 #include "common/Math.h"
 
-using namespace backend;
-
 // Tests for ScanForward
 TEST(Math, ScanForward) {
     // Test extrema