Lambda syntax completion

[sirgl]

It would be great, if closure syntax (with pipes) can be completed.
E. g. for:

pub fn map<O, F: FnOnce(T) -> O>(self, mapper: F) -> Spanned<O> {
   ...
}

fn use(s: Spanned<i32>) {
   s.map(<caret>)
}

Here I expect completion item |i| <caret>.

Name suggestion already implemented for introduce variable refactoring.
Also, after completion we can suggest change parameter's name.