Use contains instead of manual implementation

Clippy emits:

  warning: manual `!RangeInclusive::contains` implementation

As suggested, use contains instead of manual implementation.

Author: tcharding

src/expression.rs

@@ -195,7 +195,7 @@ impl<'a> Tree<'a> {
 pub fn parse_num(s: &str) -> Result<u32, Error> {
     if s.len() > 1 {
         let ch = s.chars().next().unwrap();
-        if ch < '1' || ch > '9' {
+        if !('1'..='9').contains(&ch) {
             return Err(Error::Unexpected(
                 "Number must start with a digit 1-9".to_string(),
             ));