Brainstorm how to generate GObject Introspection data automatically, plus C header files

It should be possible to make our procedural macro generate GObject Introspection data (foo.gir) automatically. This will let the generated code be consumed by language bindings, like if it were written in C.

While we are at it, it should also be possible to generate C header files for the generated GObject API.

I'm not sure how this should work. Should this be specified in the same place as the gobject_gen! invocation? (Is it possible to specify attributes before a proc macro invocation?)

#[generate_gir("foo.gir")]
#[generate_c_header("foo.h")]
gobject_gen! {
    "foo.gir",
    "foo.h",

    class Foo {
        ...
    }

    impl Foo {
        ...
    }
}