Resolve get_first clippy lint

    error: accessing first element with `slice.as_bytes().get(0)`
       --> src/error.rs:441:11
        |
    441 |     match slice.as_bytes().get(0) {
        |           ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `slice.as_bytes().first()`
        |
        = note: `-D clippy::get-first` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

src/error.rs

@@ -438,7 +438,7 @@ fn parse_line_col(msg: &mut String) -> Option<(usize, usize)> {
 }
 
 fn starts_with_digit(slice: &str) -> bool {
-    match slice.as_bytes().get(0) {
+    match slice.as_bytes().first() {
         None => false,
         Some(&byte) => byte >= b'0' && byte <= b'9',
     }