Avoid small allocations when parsing short hex format

The `.repeat()` calls allocate a `String`, instead it's possible to
parse the single digit into an `u8` and then repeat the hex value with
bitwise operations.

src/parser/mod.rs

@@ -349,16 +349,19 @@ fn parse_hex(s: &str) -> Result<Color, ParseColorError> {
 
     let n = s.len();
 
+    fn parse_single_digit(digit: &str) -> Result<u8, ParseColorError> {
+        u8::from_str_radix(digit, 16)
+            .map(|n| (n << 4) | n)
+            .map_err(|_| ParseColorError::InvalidHex)
+    }
+
     if n == 3 || n == 4 {
-        let r =
-            u8::from_str_radix(&s[0..1].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?;
-        let g =
-            u8::from_str_radix(&s[1..2].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?;
-        let b =
-            u8::from_str_radix(&s[2..3].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?;
+        let r = parse_single_digit(&s[0..1])?;
+        let g = parse_single_digit(&s[1..2])?;
+        let b = parse_single_digit(&s[2..3])?;
 
         let a = if n == 4 {
-            u8::from_str_radix(&s[3..4].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?
+            parse_single_digit(&s[3..4])?
         } else {
             255
         };