refactor(lexer): fix warnings by 'cargo clippy' for lexer (#1000)

Signed-off-by: Abhishek Kumar <[email protected]>

kclvm/lexer/src/number.rs

@@ -66,7 +66,7 @@ impl<'a> Cursor<'a> {
                 // with a number
                 self.bump();
                 let mut empty_exponent = false;
-                if self.peek().is_digit(10) {
+                if self.peek().is_ascii_digit() {
                     self.eat_decimal_digits();
                     match self.peek() {
                         'e' | 'E' => {

kclvm/lexer/src/tests.rs

@@ -1,10 +1,12 @@
 use super::*;
 use expect_test::{expect, Expect};
+use std::fmt::Write;
 
 fn check_lexing(src: &str, expect: Expect) {
-    let actual: String = tokenize(src)
-        .map(|token| format!("{:?}\n", token))
-        .collect();
+    let actual: String = tokenize(src).fold(String::new(), |mut acc, token| {
+        writeln!(acc, "{:?}", token).expect("Failed to write to string");
+        acc
+    });
     expect.assert_eq(&actual)
 }