Added a generic reallocation implementation based on ei_aligned_malloc/_free.

Rewrote ei_handmade_aligned_realloc such that it is now using std::realloc.
Reorganized functions in Memory.h for better readability.
Add missing <cerrno> include to Core (it's now required in Memory.h).

Eigen/Core

@@ -114,6 +114,7 @@
   #endif
 #endif
 
+#include <cerrno>
 #include <cstdlib>
 #include <cmath>
 #include <complex>