Suppress some new dead_code warnings

Bisects to rust-lang/rust#141407.

    warning: function `unindent` is never used
     --> src/unindent.rs:3:8
      |
    3 | pub fn unindent(s: &str) -> String {
      |        ^^^^^^^^
      |
      = note: `#[warn(dead_code)]` on by default

    warning: function `unindent_bytes` is never used
      --> src/unindent.rs:10:8
       |
    10 | pub fn unindent_bytes(s: &[u8]) -> Vec<u8> {
       |        ^^^^^^^^^^^^^^

    warning: trait `Unindent` is never used
      --> src/unindent.rs:53:11
       |
    53 | pub trait Unindent {
       |           ^^^^^^^^

Author: dtolnay

src/lib.rs

@@ -134,6 +134,7 @@
 
 mod error;
 mod expr;
+#[allow(dead_code)]
 mod unindent;
 
 use crate::error::{Error, Result};