api: tweak the lifetime of Captures::get_match

This was incredibly subtle and easy to miss. I actually didn't see any
warnings for this until I did `cargo publish` for the `1.12.0` release.
Not sure why.

I'll have to yank the `1.12.0` release because this is technically a
breaking change from the API published in `1.12.0`. Sigh. In particular,
in `1.12.0`, I assumed the inferred lifetime is tied to the borrow of
`Captures`, which is less than or equal to the lifetime of the haystack
`'h`.

Author: BurntSushi

src/regex/bytes.rs

@@ -1681,7 +1681,7 @@ impl<'h> Captures<'h> {
     /// assert_eq!(caps.get_match().as_bytes(), b"abc123");
     /// ```
     #[inline]
-    pub fn get_match(&self) -> Match {
+    pub fn get_match(&self) -> Match<'h> {
         self.get(0).unwrap()
     }
 

src/regex/string.rs

@@ -1692,7 +1692,7 @@ impl<'h> Captures<'h> {
     ///
     /// ```
     #[inline]
-    pub fn get_match(&self) -> Match {
+    pub fn get_match(&self) -> Match<'h> {
         self.get(0).unwrap()
     }