Forbid storing an `impl Trait` inside types

[CohenArthur]

struct BinOpInvalid {
    lhs: i32,
    rhs: i32,
    f: impl Fn(i32) -> i32, // { dg-error "impl Trait not allowed here" }
}

[philberty]

This is blocked by me waiting on my opaque types branch

[powerboat9]

@philberty is it still blocked? I'm looking for old issues to note resolved/fix