Introduce #[trait_alias] macro to simplify definition of alias traits

[soareschen]

Summary

This PR introduces a new #[trait_alias] macro, that can be used to simplify the definition of alias traits with blanket implementations. Using the macro, the blanket implementation would be derived automatically, thereby saving up to 50% or more code for the trait alias definition.

Example

The HasAsyncErrorType trait is now defined as follows:

#[trait_alias]
pub trait HasAsyncErrorType: Async + HasErrorType<Error: Async> {}

The macro now expands the following blanket implementation automatically:

impl<Context> HasAsyncErrorType for Context 
where
    Context: Async + HasErrorType<Error: Async> {}

Associated Items

The #[trait_alias] macro can also be used to automatically derive associated types and methods. For example:

#[trait_alias]
pub trait HasFooAtBar: HasFooAt<Bar, Foo = Self::FooBar> {
    type FooBar: Clone;
}

would generate the blanket impl:

impl<Context, FooBar> HasFooAtBar for Context
where
    Context: HasFooAt<Bar, Foo = FooBar>,
    FooBar: Clone,
{
    type FooBar = FooBar;
}

And given the following:

#[trait_alias]
pub trait CanDoFooBar: CanDoFoo + CanDoBar {
    fn foo_bar(&self) {
        self.foo().bar();
    }
}

Would be desugared into:

pub trait CanDoFooBar: CanDoFoo + CanDoBar {
    fn foo_bar(&self);
}

impl<Context> CanDoFooBar for Context
where
    Context: CanDoFoo + CanDoBar,
{
    fn foo_bar(&self) {
        self.foo().bar();
    }
}