refactor(kernel/formatter): hide 'unsafe' constructor

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

src/frontends/lean/pp.cpp

@@ -1373,7 +1373,7 @@ public:
 };
 
 formatter mk_pp_formatter(frontend const & fe) {
-    return formatter(new pp_formatter_cell(fe));
+    return mk_formatter(pp_formatter_cell(fe));
 }
 
 std::ostream & operator<<(std::ostream & out, frontend const & fe) {

src/kernel/formatter.cpp

@@ -33,6 +33,6 @@ public:
     }
 };
 formatter mk_simple_formatter() {
-    return formatter(new simple_formatter_cell());
+    return mk_formatter(simple_formatter_cell());
 }
 }

src/kernel/formatter.h

@@ -40,15 +40,20 @@ public:
 */
 class formatter {
     std::shared_ptr<formatter_cell> m_cell;
-public:
     formatter(formatter_cell * c):m_cell(c) {}
-    formatter(std::shared_ptr<formatter_cell> const & c):m_cell(c) {}
+public:
     format operator()(expr const & e, options const & opts = options()) const { return (*m_cell)(e, opts); }
     format operator()(context const & c, options const & opts = options()) const { return (*m_cell)(c, opts); }
     format operator()(context const & c, expr const & e, bool format_ctx = false, options const & opts = options()) const { return (*m_cell)(c, e, format_ctx, opts); }
     format operator()(object const & obj, options const & opts = options()) const { return (*m_cell)(obj, opts); }
     format operator()(environment const & env, options const & opts = options()) const { return (*m_cell)(env, opts); }
+
+    template<typename FCell>
+    friend formatter mk_formatter(FCell && fcell) {
+        return formatter(new FCell(std::forward<FCell>(fcell)));
+    }
 };
+
 /**
    \brief Create a simple formatter object based on \c print function.
 */