Introduce #[cgp_inherit] for inheriting from presets

[soareschen]

Summary

With #175 deprecating #[cgp_context], we need a different way for concrete contexts to inherit from a preset. This PR introduces a new #[cgp_inherit] macro that can be used for a context to inherit from a preset.

Example

Given the following:

#[cgp_inherit(MyPreset)]
pub struct App { ... }

The following will be generated:

impl<Name> DelegateComponent<Name> for App
where
    Self: MyPreset::IsPreset<Name>,
    MyPreset::Components: DelegateComponent<Name>,
{
    type Delegate = <MyPreset::Components as DelegateComponent<Name>>::Delegate;
}
impl<Name, Context, Params> IsProviderFor<Name, Context, Params>
    for App
where
    Self: MyPreset::IsPreset<Name>,
    MyPreset::Components: IsProviderFor<Name, Context, Params>,
{
}

Note that even with the blanket implementation of DelegateComponent, we can still implement consumer traits for App if the given component key is not found in the preset.