Not defining USE_DSTRING breaks cbmc

[romainbrenguier]

Commenting out #define USE_DSTRING means cbmc does not compile.
The possibility of commenting this does not seem to be used, and has not been tested for some time.
We should determine whether it's useful to have this possibility and if yes fix the compilation problems.

[  5%] Building CXX object lib/cbmc/src/util/CMakeFiles/util.dir/array_name.cpp.o
In file included from cbmc/src/util/std_types.h:19:0,
                 from cbmc/src/util/arith_tools.cpp:15:
cbmc/src/util/expr.h:181:14: error: ‘list’ in namespace ‘std’ does not name a template type
 typedef std::list<exprt> expr_listt;
              ^
In file included from cbmc/src/util/arith_tools.cpp:15:0:
cbmc/src/util/std_types.h:903:10: error: ‘unordered_map’ in namespace ‘std’ does not name a template type
     std::unordered_map<irep_idt, std::size_t, irep_id_hash> parameter_indicest;
          ^
/home/romain/git/test-gen/lib/cbmc/src/util/std_types.h:906:3: error: ‘parameter_indicest’ does not name a type
   parameter_indicest parameter_indices() const
   ^
cbmc/src/util/CMakeFiles/util.dir/build.make:62: recipe for target 'cbmc/src/util/CMakeFiles/util.dir/arith_tools.cpp.o' failed
make[3]: *** [lib/cbmc/src/util/CMakeFiles/util.dir/arith_tools.cpp.o] Error 1
make[3]: *** Waiting for unfinished jobs....
In file included from cbmc/src/util/array_name.cpp:14:0:
cbmc/src/util/expr.h:181:14: error: ‘list’ in namespace ‘std’ does not name a template type
 typedef std::list<exprt> expr_listt;
              ^
In file included from cbmc/src/util/std_expr.h:21:0,
                 from bmc/src/util/ssa_expr.h:13,
                 from cbmc/src/util/array_name.cpp:17:
cbmc/src/util/std_types.h:903:10: error: ‘unordered_map’ in namespace ‘std’ does not name a template type
     std::unordered_map<irep_idt, std::size_t, irep_id_hash> parameter_indicest;
          ^
cbmc/src/util/std_types.h:906:3: error: ‘parameter_indicest’ does not name a type
   parameter_indicest parameter_indices() const
   ^
lib/cbmc/src/util/CMakeFiles/util.dir/build.make:86: recipe for target 'lib/cbmc/src/util/CMakeFiles/util.dir/array_name.cpp.o' failed
make[3]: *** [lib/cbmc/src/util/CMakeFiles/util.dir/array_name.cpp.o] Error 1
CMakeFiles/Makefile2:1307: recipe for target 'lib/cbmc/src/util/CMakeFiles/util.dir/all' failed
make[2]: *** [lib/cbmc/src/util/CMakeFiles/util.dir/all] Error 2
CMakeFiles/Makefile2:1814: recipe for target 'lib/cbmc/src/cbmc/CMakeFiles/cbmc.dir/rule' failed
make[1]: *** [lib/cbmc/src/cbmc/CMakeFiles/cbmc.dir/rule] Error 2
Makefile:532: recipe for target 'cbmc' failed
make: *** [cbmc] Error 2

[tautschnig]

Just a few notes until time for a proper fix becomes available:

1. expr_listt has a very small number of users and should be declared locally rather than imposing the list header on each and every file.
2. "std_types.h" needs #include <unordered_map> (since 3f1fd64).

And then a lot more fixes are necessary ... working on it.