In BTreeMap::eq, do not compare the elements if the sizes are different.

Reverts 68a7c25078 in library/

(cherry picked from commit 4adcdbb58b)

library/alloc/src/collections/btree/map.rs

@@ -2414,7 +2414,7 @@ fn default() -> BTreeMap<K, V> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K: PartialEq, V: PartialEq, A: Allocator + Clone> PartialEq for BTreeMap<K, V, A> {
     fn eq(&self, other: &BTreeMap<K, V, A>) -> bool {
-        self.iter().eq(other)
+        self.len() == other.len() && self.iter().zip(other).all(|(a, b)| a == b)
     }
 }