lint: make clippy happy

Replaces `get(0)` with `first()` as suggested by https://rust-lang.github.io/rust-clippy/master/index.html#get_first

Author: sethp

src/util.rs

@@ -232,15 +232,15 @@ pub trait ToSanitizedCase {
 impl ToSanitizedCase for str {
     fn to_sanitized_pascal_case(&self) -> Cow<str> {
         let s = Case::Pascal.sanitize(self);
-        if s.as_bytes().get(0).unwrap_or(&0).is_ascii_digit() {
+        if s.as_bytes().first().unwrap_or(&0).is_ascii_digit() {
             Cow::from(format!("_{}", s))
         } else {
             s
         }
     }
     fn to_sanitized_constant_case(&self) -> Cow<str> {
         let s = Case::Constant.sanitize(self);
-        if s.as_bytes().get(0).unwrap_or(&0).is_ascii_digit() {
+        if s.as_bytes().first().unwrap_or(&0).is_ascii_digit() {
             Cow::from(format!("_{}", s))
         } else {
             s
@@ -250,7 +250,7 @@ impl ToSanitizedCase for str {
         const INTERNALS: [&str; 4] = ["set_bit", "clear_bit", "bit", "bits"];
 
         let s = Case::Snake.sanitize(self);
-        if s.as_bytes().get(0).unwrap_or(&0).is_ascii_digit() {
+        if s.as_bytes().first().unwrap_or(&0).is_ascii_digit() {
             format!("_{}", s).into()
         } else if INTERNALS.contains(&s.as_ref()) {
             s + "_"