Provide Vec::dedup_by in addition to Vec::dedup

[xrl]

I would like to dedup a Vec of structs using a property of the struct. I do not want to map since I want to maintain the contents of the Vec. The stdlib Vec can sort_by using a comparator F: FnMut(&T, &T) -- it would be consistent for Vec to dedup_by using the same kind of comparator.

My contrived example:

#[derive(PartialEq)]
struct CoolThing( pub &'static str, pub u8 );

fn main(){
    let mut things = vec![
      CoolThing("Jimbo", 10), CoolThing("Billy", 10)
    ];

    let uniq_things = things.dedup_by(|x| x.1);
    assert!( vec![CoolThing("Billy",10)] == uniq_things )
}

[mbrubeck]

Fixed by rust-lang/rust#36743.