Make EcoVec::extend_from_trusted public

Add the bound `I::IntoIter: ExactSizeIterator` to try to prevent unsafe usage.

src/vec.rs

@@ -686,10 +686,12 @@ impl<T> EcoVec<T> {
 impl<T: Clone> EcoVec<T> {
     /// Clones and pushes all elements in a trusted-len iterator to the vector.
     ///
+    /// # Safety
     /// The iterator must produce exactly `count` items.
-    unsafe fn extend_from_trusted<I>(&mut self, count: usize, iter: I)
+    pub unsafe fn extend_from_trusted<I>(&mut self, count: usize, iter: I)
     where
         I: IntoIterator<Item = T>,
+        I::IntoIter: ExactSizeIterator,
     {
         if count == 0 {
             return;