Use qualified path for Result::Ok in bincode_derive

[nhaef]

fixes #756 by using a qualified path for core::result::Result::Ok.

[VictorKoenders]

Can you add a test for this too? Something like:

fn Ok(){}

#[derive(Encode, Decode)]
struct A { a: u32 }

#[derive(Encode, Decode)]
enum B { A, B }