Automatically box use of struct as a field in that struct

[ctaggart]

As hinted in #84, we should (when possible) automatically box types when they are used as the type of their own field.

For example, it should be possible to detect the following situation:

struct Foo {
   bar: Bar,
   foo: Foo
}

and turn it into:

struct Foo {
   bar: Bar,
   foo: Box<Foo>
}

[cataggar]

Fixed by #1385.