Support implicit Context type in #[cgp_impl]

[soareschen]

This PR brings a quality of life improvement on the use of #[cgp_impl], by allowing the generic Context type to be omitted.

Example

Before this PR, the generic Context needs to be always specified, such as:

#[cgp_impl(GreetHello)]
impl<Context> Greeter for Context
where
    Context: HasName,
{
    fn greet(&self) {
        println!("Hello, {}!", self.name());
    }
}

With the changes in this PR, the same provider can now be simplified as:

#[cgp_impl(GreetHello)]
impl Greeter
where
    Self: HasName,
{
    fn greet(&self) {
        println!("Hello, {}!", self.name());
    }
}

This significantly improves the ergonomic of writing context-generic implementations in the simple cases. With the generic Context type removed, a #[cgp_impl] block looks much more like a direct implementation of a trait with no generic parameters involved. This can bridge the gap for developers coming from OOP background, as they can view the implementation as being similar to implementing a class.