Closure support

[hajifkd]

Something like this;

impl<A, B> JSSerialize for Box<Box<Fn(A) -> B>> where A: JSDeserialize, B: JSSerialize {
    fn serialize(&self) -> EM_GENERIC_WIRE_TYPE {
        // equivalent for Module.helper_vi.bind(Module, Box::into_raw(Box::new(*self)))...
    }
}

We may well need JSSerializeOnce (JSSerializeFunc?)

pub trait JSSerializeOnce: JSID {
    fn serialize(self) -> EM_GENERIC_WIRE_TYPE;
}

since either into_raw or new consumes the pointer and Clone may not be used.
Inside args! macro, we can use a kind of duck typing with JSSerializeOnce.
Or, something like

macro_rules! jsfn {
    ( $func:expr ) => {{
        let boxed = Box::new(Box::new($func));
        Val::new(boxed.serialize());
    }}
}
...
jsfn!(|x| x + 10)

with JSSerializeOnce also sounds okay and rather better.

[hajifkd]

Just note: cast to trait object was necessary.

Now, we should discuss the interface.
Unlike the current implementation, for instance,

impl JSSerialize for Box<Box<Fn(....) -> ...>> {
...
}

also sounds nice.

Considering the caller problem (#5), maybe the current one with some macro would be better, isn't it?