Macro impl_callback 

macro_rules! impl_callback {
    ($callback_value:ident, $callback_ty:ty) => { ... };
}

Implements Display, Debug, Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Hash for a Callback with a .cb and .ctx field.

This is necessary to work around for https://github.com/rust-lang/rust/issues/54508