fix(library/max_sharing): take binder annotations into account

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/library/max_sharing.cpp

@@ -17,7 +17,7 @@ namespace lean {
    shared sub-expressions.
 */
 struct max_sharing_fn::imp {
-    typedef typename std::unordered_set<expr, expr_hash, std::equal_to<expr>> expr_cache;
+    typedef typename std::unordered_set<expr, expr_hash, is_bi_equal_proc> expr_cache;
 
     expr_cache m_cache;