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). On SV-COMP's ReachSafety-ECA with this change we can
perform 3819.81 symex_step calls per second, compared to 2752.28 calls
per second with the std::map configuration. The performance improvements
are spread across various `irept`-related operations.

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>