Compact yaml-string-quoting code a bit

In the hopes that this is also faster.

src/main.rs

@@ -53,6 +53,7 @@ fn quote_yaml_string_if_needed(string: &str) -> String {
     let mut is_number = true;
     let mut is_hex_number = false;
     let mut no_number_separator_yet = true;
+    let starts_with_0x = string.starts_with("0x");
     for c in string.chars() {
         match c {
             '0'..='9' => (),
@@ -61,7 +62,7 @@ fn quote_yaml_string_if_needed(string: &str) -> String {
                 no_number_separator_yet = false;
                 is_hex_number = false;
             }
-            'x' if string.starts_with("0x") && no_number_separator_yet => {
+            'x' if starts_with_0x && no_number_separator_yet => {
                 no_number_separator_yet = false;
                 is_hex_number = true;
             }
@@ -78,8 +79,7 @@ fn quote_yaml_string_if_needed(string: &str) -> String {
     if is_number
         || string.starts_with(special_start_chars)
         || string.contains(": ")
-        || string.contains(|c: char| matches!(c, '\0'..='\x1f'))
-        || string.contains('"')
+        || string.contains(|c: char| matches!(c, '\0'..='\x1f' | '"'))
         || string.ends_with([' ', ':'])
         || ["yes", "no", "true", "false", "on", "off", "null", "~"]
             .contains(&string.to_lowercase().as_str())