Eliminate an allocation from Literal::byte_string

    error: `format!(..)` appended to existing `String`
       --> src/fallback.rs:879:22
        |
    879 |                 _ => escaped.push_str(&format!("\\x{:02X}", b)),
        |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: `-D clippy::format-push-string` implied by `-D clippy::all`
        = help: consider using `write!` to avoid the extra allocation
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#format_push_string

src/fallback.rs

@@ -4,7 +4,7 @@ use crate::{Delimiter, Spacing, TokenTree};
 use std::cell::RefCell;
 #[cfg(span_locations)]
 use std::cmp;
-use std::fmt::{self, Debug, Display};
+use std::fmt::{self, Debug, Display, Write as _};
 use std::iter::FromIterator;
 use std::mem;
 use std::ops::RangeBounds;
@@ -876,7 +876,9 @@ impl Literal {
                 b'"' => escaped.push_str("\\\""),
                 b'\\' => escaped.push_str("\\\\"),
                 b'\x20'..=b'\x7E' => escaped.push(*b as char),
-                _ => escaped.push_str(&format!("\\x{:02X}", b)),
+                _ => {
+                    let _ = write!(escaped, "\\x{:02X}", b);
+                }
             }
         }
         escaped.push('"');