Introduce #[cgp_impl] to simplify provider trait implementation

[soareschen]

Summary

This PR introduces a new #[cgp_impl] macro that replaces the #[cgp_provider] macro and provides much better ergonomics for implementing provider traits in CGP.

In essence, #[cgp_impl] allows you to write a provider trait implementation as if it is a blanket implementation for the consumer trait. With this, the implementation of CGP providers become as intuitive as just implementing regular Rust traits.

Example

Consider the following example trait:

#[cgp_component(HttpRequestFetcher)]
pub trait CanFetchHttpRequest: HasErrorType {
    fn fetch_http_request(&self, request: Request) -> Result<Response, Self::Error>;
}

With #[cgp_impl], you can now implement a provider such as follows:

#[cgp_impl(new FetchWithHttpClient)]
impl<Context> HttpRequestFetcher for Context
where
    Self: HasHttpClient + HasErrorType,
{
    fn fetch_http_request(&self, request: Request) -> Result<Response, Self::Error> {
        ...
    }
}

Compared to before, you would have to implement the provider using #[cgp_provider] as follows:

#[cgp_new_provider]
impl<Context> HttpRequestFetcher<Context> for FetchWithHttpClient
where
    Context: HasHttpClient + HasErrorType,
{
    fn fetch_http_request(context: &Context, request: Request) -> Result<Response, Context::Error> {
        todo!()
    }
}

As we can see, the ergonomic for #[cgp_impl] is much closer to vanilla Rust trait implementation.

Behind the scene, #[cgp_impl] desugars the provider trait implementation to the same as how you would manually write it using #[cgp_provider]. So understanding of provider traits is still necessary, especially during debugging.