make_widget → impl_singleton

[dhardy]

What was make_widget is made more generic, no longer being special to widgets (aside from a rule for #[widget] fields which adds the Widget trait bound):

• do not add widget_core!() field or derive Debug automatically
• construct values using Default::default() if omitted
• make locally-declared type parameter names unique
• rename to impl_singleton
• support unit and tuple structs