Support underscore constants

[dtolnay]

As introduced in rust 1.37: https://blog.rust-lang.org/2019/08/15/Rust-1.37.0.html#using-unnamed-const-items-for-macros

Iota currently rejects code containing underscore names.

iota! {
    const A: u8 = 1 << iota;
        | _
        | C
}

error: no rules expected the token `|`
 --> src/main.rs:5:9
  |
5 |         | _
  |         ^ no rules expected this token in macro call