map: Integrate color into lowest bit of parent pointer

For the implementation of a red-black tree, we can combine the parent
pointer and color as what Linux kernel does -- inject the color info
in the lowest bit of the parent pointer, resulting in better locaity.

common.h

@@ -8,6 +8,15 @@
 #define unlikely(x) x
 #endif
 
+/* Alignment macro */
+#if defined(__GNUC__) || defined(__clang__)
+#define __ALIGNED(x) __attribute__((aligned(x)))
+#elif defined(_MSC_VER)
+#define __ALIGNED(x) __declspec(align(x))
+#else /* unspported compilers */
+#define __ALIGNED(x)
+#endif
+
 /* Pattern Matching for C macros.
  * https://github.com/pfultz2/Cloak/wiki/C-Preprocessor-tricks,-tips,-and-idioms
  */