Add #[impl_generics] attribute to #[cgp_fn]

[soareschen]

This PR introduces a new #[impl_generics] attribute that can be used inside #[cgp_fn]. This allows the specified generic parameters to only be included in the generated impl, but not in the trait definition.

This allows the CGP function to bind generic fields using #[impl_generics] without polluting the trait with generic parameters.

For example:

#[cgp_fn]
#[impl_generics(Name: Display)]
pub fn greet(&self, #[implicit] name: &Name) -> String {
    format!("Hello, {}!", name)
}

#[cgp_fn]
#[uses(Greet)]
pub fn print_greet(&self) {
    println!("{}", self.greet());
}