automata: make Input::haystack return with original lifetime

This was an oversight in the API. Since shared borrows use
sub-typing, this should be a fully backward compatible change.

PR #1277

Author: Chaoses-Ib

regex-automata/src/util/search.rs

@@ -590,7 +590,7 @@ impl<'h> Input<'h> {
     /// assert_eq!(b"foobar", input.haystack());
     /// ```
     #[inline]
-    pub fn haystack(&self) -> &[u8] {
+    pub fn haystack(&self) -> &'h [u8] {
         self.haystack
     }