Fix ide-assist: generate Deref transitive

[A4-Tacks]

Example

struct Foo<T>($0T);

Current output:

struct Foo<T>(T);

impl<T: std::ops::Deref> std::ops::Deref for Foo<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

This PR fixed output:

struct Foo<T>(T);

impl<T> std::ops::Deref for Foo<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

[lnicola]

Do we need to keep the old function?

[A4-Tacks]

Will deleting this function affect other PRs?

[lnicola]

I don't think so, but ok.