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/util.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::str::FromStr;
 
 use super::compare_field::compare_property_list_of_quoted_string;
@@ -206,10 +207,10 @@ pub(crate) fn get_property_unquoted_atom<'s>(
 /// Get a named property containing an quoted string from the emacs token.
 ///
 /// Returns None if key is not found.
-pub(crate) fn get_property_quoted_string(
-    emacs: &Token<'_>,
+pub(crate) fn get_property_quoted_string<'s>(
+    emacs: &Token<'s>,
     key: &str,
-) -> Result<Option<String>, Box<dyn std::error::Error>> {
+) -> Result<Option<Cow<'s, str>>, Box<dyn std::error::Error>> {
     get_property(emacs, key)?
         .map(Token::as_atom)
         .map_or(Ok(None), |r| r.map(Some))?