Use std::forward_list instead of std::map in irept by default

This reduces the size of an irept by 5 pointers (i.e., 40 bytes on
64-bit systems).

Also make typet constructor compatible with the STL shipped with GCC 5.

Author: tautschnig

src/util/irep.h

@@ -18,7 +18,7 @@ Author: Daniel Kroening, [email protected]
 
 #define SHARING
 // #define HASH_CODE
-// #define NAMED_SUB_IS_FORWARD_LIST
+#define NAMED_SUB_IS_FORWARD_LIST
 
 #ifdef NAMED_SUB_IS_FORWARD_LIST
 #include <forward_list>

src/util/type.h

@@ -32,10 +32,18 @@ class typet:public irept
 
   explicit typet(const irep_idt &_id):irept(_id) { }
 
+#if !defined(__GNUC__) || __GNUC__ >= 6
   typet(irep_idt _id, typet _subtype)
     : irept(std::move(_id), {}, {std::move(_subtype)})
   {
   }
+#else
+  typet(irep_idt _id, typet _subtype)
+    : irept(std::move(_id))
+  {
+    subtype() = std::move(_subtype);
+  }
+#endif
 
   const typet &subtype() const
   {