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

.travis.yml

@@ -177,7 +177,7 @@ jobs:
         - CCACHE_CPP2=yes
       script: echo "Not running any tests for a debug build."
 
-    # cmake build using g++-7, enable NAMED_SUB_IS_FORWARD_LIST
+    # cmake build using g++-7, disable NAMED_SUB_IS_FORWARD_LIST
     - stage: Test different OS/CXX/Flags
       os: linux
       sudo: false
@@ -200,7 +200,7 @@ jobs:
       install:
         - ccache -z
         - ccache --max-size=1G
-        - cmake -H. -Bbuild '-DCMAKE_BUILD_TYPE=Release' '-DCMAKE_CXX_COMPILER=/usr/bin/g++-7' '-DCMAKE_CXX_FLAGS=-DNAMED_SUB_IS_FORWARD_LIST'
+        - cmake -H. -Bbuild '-DCMAKE_BUILD_TYPE=Release' '-DCMAKE_CXX_COMPILER=/usr/bin/g++-7' '-DCMAKE_CXX_FLAGS=-DNAMED_SUB_IS_FORWARD_LIST=0'
         - git submodule update --init --recursive
         - cmake --build build -- -j4
       script: (cd build; bin/unit "[core][irept]")

src/util/irep.h

@@ -18,7 +18,9 @@ Author: Daniel Kroening, kroening@kroening.com
 
 #define SHARING
 // #define HASH_CODE
-// #define NAMED_SUB_IS_FORWARD_LIST
+#ifndef NAMED_SUB_IS_FORWARD_LIST
+#  define NAMED_SUB_IS_FORWARD_LIST
+#endif
 
 #ifdef NAMED_SUB_IS_FORWARD_LIST
 #include <forward_list>