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/Math.h

@@ -18,26 +18,22 @@
 #include <cstddef>
 #include <cstdint>
 
-namespace backend {
+// The following are not valid for 0
+uint32_t ScanForward(uint32_t bits);
+uint32_t Log2(uint32_t value);
+bool IsPowerOfTwo(size_t n);
 
-    // The following are not valid for 0
-    uint32_t ScanForward(uint32_t bits);
-    uint32_t Log2(uint32_t value);
-    bool IsPowerOfTwo(size_t n);
+bool IsAligned(const void* ptr, size_t alignment);
+void* AlignVoidPtr(void* ptr, size_t alignment);
 
-    bool IsAligned(const void* ptr, size_t alignment);
-    void* AlignVoidPtr(void* ptr, size_t alignment);
-
-    template<typename T>
-    T* Align(T* ptr, size_t alignment) {
-        return reinterpret_cast<T*>(AlignVoidPtr(ptr, alignment));
-    }
-
-    template<typename T>
-    const T* Align(const T* ptr, size_t alignment) {
-        return reinterpret_cast<const T*>(AlignVoidPtr(const_cast<T*>(ptr), alignment));
-    }
+template<typename T>
+T* Align(T* ptr, size_t alignment) {
+    return reinterpret_cast<T*>(AlignVoidPtr(ptr, alignment));
+}
 
+template<typename T>
+const T* Align(const T* ptr, size_t alignment) {
+    return reinterpret_cast<const T*>(AlignVoidPtr(const_cast<T*>(ptr), alignment));
 }
 
 #endif // COMMON_MATH_H_