core/char: Add comment to to_digit()

Author: Turbo87

Diff for: src/libcore/char/methods.rs

@@ -122,6 +122,9 @@ impl char {
     #[inline]
     pub fn to_digit(self, radix: u32) -> Option<u32> {
         assert!(radix <= 36, "to_digit: radix is too high (maximum 36)");
+
+        // the code is split up here to improve execution speed for cases where
+        // the `radix` is constant and 10 or smaller
         let val = if radix <= 10  {
             match self {
                 '0' ..= '9' => self as u32 - '0' as u32,