Improve list iterator

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

src/frontend/pp.cpp

@@ -303,7 +303,7 @@ class pp_fn {
                 auto it = parts.begin();
                 for (unsigned i = 1; i < num_args(e); i++) {
                     result p_arg = pp_mixfix_child(op, arg(e, i), depth);
-                    unsigned sz  = (*it).size();
+                    unsigned sz  = it->size();
                     r_format    += format{format(*it), nest(sz+1, format{line(), p_arg.first})};
                     r_depth      = std::max(r_depth, p_arg.second);
                     ++it;

src/util/list.h

@@ -82,6 +82,7 @@ public:
         bool operator==(iterator const & s) const { return m_it == s.m_it; }
         bool operator!=(iterator const & s) const { return !operator==(s); }
         T const & operator*() { lean_assert(m_it); return m_it->m_head; }
+        T const * operator->() { lean_assert(m_it); return &(m_it->m_head); }
     };
 
     iterator begin() const { return iterator(m_ptr); }