Use btree iterator subtraction instead of std::distance in erase_range() and count().

PiperOrigin-RevId: 481979737
Change-Id: I69f53665b0463a7d8d80f2a3feedfdd95d32b012

absl/container/internal/btree.h

@@ -2444,7 +2444,7 @@ auto btree<P>::rebalance_after_delete(iterator iter) -> iterator {
 template <typename P>
 auto btree<P>::erase_range(iterator begin, iterator end)
     -> std::pair<size_type, iterator> {
-  size_type count = static_cast<size_type>(std::distance(begin, end));
+  size_type count = static_cast<size_type>(end - begin);
   assert(count >= 0);
 
   if (count == 0) {

absl/container/internal/btree_container.h

@@ -107,7 +107,7 @@ class btree_container {
   template <typename K = key_type>
   size_type count(const key_arg<K> &key) const {
     auto equal_range = this->equal_range(key);
-    return std::distance(equal_range.first, equal_range.second);
+    return equal_range.second - equal_range.first;
   }
   template <typename K = key_type>
   iterator find(const key_arg<K> &key) {