Serde support for ToLua

[tailhook]

Hi, I want to implement support for ToLua for T:Serialze from serde. So you can lua_push just about anything :)

The problem I encountered is:

error[E0119]: conflicting implementations of trait `wrapper::convert::ToLua` for type `std::option::Option<unsafe extern "C" fn(*mut libc::c_void) -> i32>`:

Can we remove that implementation and make a newtype for it? The alternative is to make a wrapper for serde:

lua.push(SerdeWrapper(object))

Which, might be good too, but I still think it's superfluous. What do you think?

[tailhook]

Another question is that serialization can fail. Should ToLua panic when it fails? Or maybe we could change ToLua::to_lua to allow errors?