lint: fix clippy get_first

src/arch/all/rabinkarp.rs

@@ -86,7 +86,7 @@ impl Finder {
     #[inline]
     pub fn new(needle: &[u8]) -> Finder {
         let mut s = Finder { hash: Hash::new(), hash_2pow: 1 };
-        let first_byte = match needle.get(0) {
+        let first_byte = match needle.first() {
             None => return s,
             Some(&first_byte) => first_byte,
         };

src/arch/all/twoway.rs

@@ -323,7 +323,7 @@ impl FinderRev {
         let nlen = needle.len();
         let mut pos = haystack.len();
         let mut shift = nlen;
-        let first_byte = match needle.get(0) {
+        let first_byte = match needle.first() {
             None => return Some(pos),
             Some(&first_byte) => first_byte,
         };
@@ -364,7 +364,7 @@ impl FinderRev {
     ) -> Option<usize> {
         let nlen = needle.len();
         let mut pos = haystack.len();
-        let first_byte = match needle.get(0) {
+        let first_byte = match needle.first() {
             None => return Some(pos),
             Some(&first_byte) => first_byte,
         };