Remove custom extend_from_slice implementation.

This was added because regex 0.1 supports Rust 1.3+. But we can now
assume Rust 1.12+, which has Vec::extend_from_slice. Yay for less unsafe!

src/re_bytes.rs

@@ -493,11 +493,11 @@ impl Regex {
                 if limit > 0 && i >= limit {
                     break
                 }
-                extend_from_slice(&mut new, &text[last_match..m.start()]);
-                extend_from_slice(&mut new, &*rep);
+                new.extend_from_slice(&text[last_match..m.start()]);
+                new.extend_from_slice(&rep);
                 last_match = m.end();
             }
-            extend_from_slice(&mut new, &text[last_match..]);
+            new.extend_from_slice(&text[last_match..]);
             return Cow::Owned(new);
         }
 
@@ -515,11 +515,11 @@ impl Regex {
             }
             // unwrap on 0 is OK because captures only reports matches
             let m = cap.get(0).unwrap();
-            extend_from_slice(&mut new, &text[last_match..m.start()]);
+            new.extend_from_slice(&text[last_match..m.start()]);
             rep.replace_append(&cap, &mut new);
             last_match = m.end();
         }
-        extend_from_slice(&mut new, &text[last_match..]);
+        new.extend_from_slice(&text[last_match..]);
         Cow::Owned(new)
     }
 }
@@ -980,7 +980,7 @@ impl<'a> Replacer for &'a [u8] {
 
 impl<F> Replacer for F where F: FnMut(&Captures) -> Vec<u8> {
     fn replace_append(&mut self, caps: &Captures, dst: &mut Vec<u8>) {
-        extend_from_slice(dst, &(*self)(caps));
+        dst.extend_from_slice(&(*self)(caps));
     }
 }
 
@@ -996,26 +996,10 @@ pub struct NoExpand<'r>(pub &'r [u8]);
 
 impl<'a> Replacer for NoExpand<'a> {
     fn replace_append(&mut self, _: &Captures, dst: &mut Vec<u8>) {
-        extend_from_slice(dst, self.0);
+        dst.extend_from_slice(self.0);
     }
 
     fn no_expansion<'r>(&'r mut self) -> Option<Cow<'r, [u8]>> {
         Some(Cow::Borrowed(self.0))
     }
 }
-
-/// This hopefully has the same performance characteristics as
-/// Vec::extend_from_slice (which was introduced in Rust 1.6), but works on
-/// Rust 1.3.
-///
-/// N.B. Remove this once we do a semver bump. At that point, we'll bump
-/// required Rust version to at least 1.6.
-fn extend_from_slice(dst: &mut Vec<u8>, src: &[u8]) {
-    dst.reserve(src.len());
-    let dst_len = dst.len();
-    unsafe { dst.set_len(dst_len + src.len()); }
-    let mut dst = &mut dst[dst_len..dst_len + src.len()];
-    for i in 0..src.len() {
-        dst[i] = src[i];
-    }
-}