Support pub(restricted) visibilities

[dtolnay]

iota! {
    pub(crate) const A: u8 = 1 << iota;
        | B
        | C
}

error: no rules expected the token `(`
 --> src/main.rs:4:8
  |
4 |     pub(crate) const A: u8 = 1 << iota;
  |        ^ no rules expected this token in macro call