Extend implement separately from insert_slice for microbenchmark improvement

Author: nipunn1313

lib.rs

@@ -433,10 +433,19 @@ impl<A: Array> FromIterator<A::Item> for SmallVec<A> {
 }
 
 impl<A: Array> Extend<A::Item> for SmallVec<A> {
-    #[inline]
     fn extend<I: IntoIterator<Item=A::Item>>(&mut self, iterable: I) {
-        let len = self.len();
-        self.insert_many(len, iterable);
+        let iter = iterable.into_iter();
+        let (lower_size_bound, _) = iter.size_hint();
+
+        let target_len = self.len + lower_size_bound;
+
+        if target_len > self.capacity() {
+           self.grow(target_len);
+        }
+
+        for elem in iter {
+            self.push(elem);
+        }
     }
 }