Handle recursive types

[avantgardnerio]

When attempting to codegen for my xsd representing recursive widgets in a windowing environment, I get:

error[E0072]: recursive types `Grid`, `GridRow`, `Widget` and `WidgetChoice` have infinite size
  --> src/generated/models.rs:39:1
   |
39 | pub struct Grid {
   | ^^^^^^^^^^^^^^^
40 |     #[yaserde(rename = "children")]
41 |     pub children: GridRow,
   |                   ------- recursive without indirection
...
55 | pub struct GridRow {
   | ^^^^^^^^^^^^^^^^^^
56 |     #[yaserde(rename = "child")]
57 |     pub child: Widget,
   |                ------ recursive without indirection
...
78 | pub enum WidgetChoice {
   | ^^^^^^^^^^^^^^^^^^^^^
79 |     Grid(Grid),
   |          ---- recursive without indirection
...
94 | pub struct Widget {
   | ^^^^^^^^^^^^^^^^^
95 |     #[yaserde(flatten)]
96 |     pub widget_choice: WidgetChoice,
   |                        ------------ recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
41 ~     pub children: Box<GridRow>,

I'm happy to start contributing, but a little help on where to get started here would help.

It seems like we would need to generate:

impl YaSerialize for Box<Grid> {
...
}

[avantgardnerio]

I am by no means suggesting we merge without cleanup, but this fixes the problem:

#165