Resolve get_first clippy lint

    warning: accessing first element with `variants_from_header.get(0)`
      --> macro/src/load.rs:39:22
       |
    39 |     let span = match variants_from_header.get(0) {
       |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `variants_from_header.first()`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first
       = note: `-W clippy::get-first` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::get_first)]`

macro/src/load.rs

@@ -36,7 +36,7 @@ pub(crate) fn load(cx: &mut Errors, apis: &mut [Api]) {
         }
     }
 
-    let span = match variants_from_header.get(0) {
+    let span = match variants_from_header.first() {
         None => return,
         Some(enm) => enm.variants_from_header_attr.clone().unwrap(),
     };