Why does disabling a struct's postblit increase its size in memory?

[Per Nordlöw]

Why does disabling a struct's postblit increase its sizeof by one 
word?

The following holds:

```d
struct S { @disable this(this); int _; }
struct T { int _; }
static assert(S.sizeof == 16);
static assert(T.sizeof == int.sizeof);
```

. Why is this needed?