No more anonymous trait parameters

In accordance with RFC #1685,parameters in trait method declarations are no longer allowed to be anonymous.

For example, in the 2015 edition, this was allowed:

#![allow(unused_variables)]
fn main() {
trait Foo {
    fn foo(&self, u8);
}
}

In the 2018 edition, all parameters must be given an argument name (even if it's just_):

#![allow(unused_variables)]
fn main() {
trait Foo {
    fn foo(&self, baz: u8);
}
}