Handle passing functions as argument by value

[CohenArthur]

fn takes_fn(a: i32, f: impl Fn(i32) -> i32) -> i32 {
    f(a)
}

[bjorn3]

This is basically identical to #1483. impl Trait in argument position desugars to a where F: Trait with the only difference being that takes_fn::<SomeType>(/* ... */) is not allowed for impl Trait.