Use properties' declared defaults when rendering whole-type defaults

[danieleades]

Fixes #662

When typify renders a whole-type default value — a struct type's default used for its impl Default, or a parent property's default expression — properties absent from the default JSON object were filled in with Default::default(), ignoring those properties' own declared default values from the schema.

Using the schema from #662:

"separator": {
  "$ref": "#/definitions/SeparatorConfig",
  "default": {}
}

previously generated

super::SeparatorConfig {
    line_color: Default::default(),      // -> None
    line_thickness: Default::default(),  // -> 0
}

which disagrees with what deserializing the declared default value {} produces (the per-property serde default functions yield Some("#B2000000") / 1). So the rendered default did not mean what the schema says it means.

With this change, a property absent from the whole-type default is rendered from its own declared default when it has one:

super::SeparatorConfig {
    line_color: ::std::option::Option::Some("#B2000000".to_string()),
    line_thickness: 1_i64,
}

falling back to Default::default() only when the property declares no default. While here, the fallback is fully qualified as ::std::default::Default::default(), since a schema can legitimately define a type named Default.

The types-with-defaults.json golden test is extended with the SeparatorConfig/SeparatorHolder example from the issue, covering both a default: {} (all properties filled from their declared defaults) and a partial default: { "lineThickness": 5 } (the remaining properties filled from their declared defaults).