Fix `make tidy` and name what is being computed

src/libcore/char.rs

@@ -470,7 +470,8 @@ fn next(&mut self) -> Option<char> {
                 Some('{')
             }
             EscapeUnicodeState::Value => {
-                let c = from_digit(((self.c as u32) >> (self.hex_digit_idx * 4)) & 0xf, 16).unwrap();
+                let hex_digit = ((self.c as u32) >> (self.hex_digit_idx * 4)) & 0xf;
+                let c = from_digit(hex_digit, 16).unwrap();
                 if self.hex_digit_idx == 0 {
                     self.state = EscapeUnicodeState::RightBrace;
                 } else {