Fix use of self inside default trait method implementation body

[soareschen]

Summary

This PR fixes a bug in #[cgp_component] when the consumer trait contains default method implementation.

Previously, this would cause compile errors in the generated code, as the provider trait would contain the default implementation that refers to self.

Example

With the fix, we can now write trait implementations such as follows:

#[cgp_component(Greeter)]
pub trait CanGreet: HasName {
    fn greet(&self) -> String {
        format!("Hello, {}!", self.name())
    }
}

which would correctly generate the following provider trait:

pub trait Greeter<Context>: IsProviderFor<GreeterComponent, Context, ()>
where
    Context: HasName,
{
    fn greet(context: &Context) -> String {
        format!("Hello, {}!", context.name())
    }
}

Previously, the following would have been generated, causing compile error with the dangling reference to self:

pub trait Greeter<Context>: IsProviderFor<GreeterComponent, Context, ()>
where
    Context: HasName,
{
    fn greet(context: &Context) -> String {
        format!("Hello, {}!", self.name())
    }
}