Fix clang++ error

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

src/kernel/normalize.h

@@ -14,7 +14,7 @@ namespace lean {
 class environment;
 /** \brief Functional object for normalizing expressions */
 class normalizer {
-    struct imp;
+    class imp;
     std::unique_ptr<imp> m_ptr;
 public:
     normalizer(environment const & env);

src/kernel/type_check.h

@@ -12,7 +12,7 @@ Author: Leonardo de Moura
 namespace lean {
 class environment;
 class type_checker {
-    struct imp;
+    class imp;
     std::unique_ptr<imp> m_ptr;
 public:
     type_checker(environment const & env);

src/util/sexpr/options.cpp

@@ -91,7 +91,7 @@ static char const * g_arrow               = "\u21a6";
 
 options options::update(name const & n, sexpr const & v) const {
     if (contains(n)) {
-        return map(m_value, [&](sexpr const & p) {
+        return map(m_value, [&](sexpr p) {
                 if (to_name(car(p)) == n)
                     return cons(car(p), v);
                 else