book: add memory alignment alignof and alignas

see #2

code/9/9.3.alignment.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+
+struct Storage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+struct alignas(std::max_align_t) AlignasStorage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+int main() {
+    std::cout << alignof(Storage) << std::endl;
+    std::cout << alignof(AlignasStorage) << std::endl;
+    return 0;
+}