Use reference to convert object to lua

[therustmonk]

Current push declaration is:

impl State {
    fn push<T: ToLua>(&mut self, value: T)
}

But ToLua satisfied with the reference &self (it doesn't need to own the instance).

trait ToLua {
    fn to_lua(&self, state: &mut State)
}

I don't see any reason to own the value inside push.
But owning makes hard to push nested structs with this generic push method, because we have convert nested items to owned (clone) to satisfy push declaration.

This PR makes tiny backward-incompatible change and uses reference only to push objects ToLua.

impl State {
    fn push<T: ToLua>(&mut self, value: &T)
}