Only allocate memory when unquoting sexp string that contains escapes.

If the quoted string contains no escape sequences, then unquoting the string can be done by simply shaving off the leading and trailing quotation marks which can be a slice operation. By returning Cow, we can return either a borrowed slice or an owned String.

src/compare/compare_field.rs

@@ -1,3 +1,5 @@
+use std::borrow::Borrow;
+use std::borrow::Cow;
 use std::collections::BTreeSet;
 use std::fmt::Debug;
 use std::str::FromStr;
@@ -262,11 +264,11 @@ pub(crate) fn compare_property_set_of_quoted_string<
         .iter()
         .map(|e| e.as_atom())
         .collect::<Result<Vec<_>, _>>()?;
-    let value: Vec<String> = value
+    let value: Vec<Cow<'_, str>> = value
         .into_iter()
         .map(unquote)
         .collect::<Result<Vec<_>, _>>()?;
-    let value: BTreeSet<&str> = value.iter().map(|e| e.as_str()).collect();
+    let value: BTreeSet<&str> = value.iter().map(|e| e.borrow()).collect();
     let mismatched: Vec<_> = value.symmetric_difference(&rust_value).copied().collect();
     if !mismatched.is_empty() {
         let this_status = DiffStatus::Bad;