Expand cgp_preset! into a preset module

[soareschen]

Summary

This PR performs some redesign on the unstable cgp_preset! macro to make the definition of presets slightly more ergonomic. The main changes is that instead of being a struct, the defined preset is now a Rust module that contains several CGP constructs related to the preset.

Use with #[cgp::re_export_imports]

The cgp_preset! macro now requires the module to be wrapped around a mod with #[cgp::re_export_imports], in order to help re-exporting all imported component constructs.

A preset is now defined in the form:

#[cgp::re_export_imports]
mod preset {
    use cgp::prelude::*;
    use foo::{FooComponent, FooProvider};
    use bar::{BarComponent, BarProvider};

    cgp_preset! {
        MyPreset {
            FooComponent: FooProvider,
            BarComponent: BarProvider,
        }
    }
}

which would be roughly expanded into:

mod preset {
    use cgp::prelude::*;
    use foo::{FooComponent, FooProvider};
    use bar::{BarComponent, BarProvider};

    mod MyPreset {
        use super::*;

        pub struct Provider;

        delegate_components! {
            Provider {
                FooComponent: FooProvider,
                BarComponent: BarProvider,
            }
        }

        pub trait IsPreset<Component> {}

        impl<T> IsPreset<FooComponent> for T {}
        impl<T> IsPreset<BarComponent> for T {}

        #[macro_export]
        macro_rules! with_my_preset {
            ($($body:tt)*) => {
                replace_with! { [FooComponent, BarComponent], $($body)* }
            };
        }

        pub use with_my_preset as with_components;

        pub mod re_exports {
            pub use super::super::super::re_exports::*;
        }
    }
}

mod re_exports {
    pub use cgp::prelude::*;
    pub use foo::{FooComponent, FooProvider};
    pub use bar::{BarComponent, BarProvider};
}

pub use preset::*;

Changes

• The provider name format has been renamed from MyPreset to MyPreset::Provider.
• The component match trait has been renamed from IsMyPreset to MyPreset::IsPreset.
• The substitution macro has been renamed from with_my_preset! to MyPreset::with_components!.