assist: replace generic with impl trait

[pickx]

fn new<P: AsRef<Path>>(location: P) -> Self { }

Placing the cursor on P: AsRef<Path> or on the P of location: P should trigger an assist to change this to

fn new(location: impl AsRef<Path>) -> Self { }

We already have an assist replace impl trait with generic that does the opposite.

[justahero]

@rustbot claim