OneVector type alias has been removed

[dtolnay]

error[E0432]: unresolved import `rustc_data_structures::small_vec`
   --> tests/test_precedence.rs:218:32
    |
218 |     use rustc_data_structures::small_vec::OneVector;
    |                                ^^^^^^^^^ Could not find `small_vec` in `rustc_data_structures`

From this change in nightly: rust-lang/rust#53824.