Fix name mangling for stress tests.

Fix stress tests such that testlib code uses the jet_ allocator, but
test code uses libjemalloc.

Generate jemalloc_{rename,mangle}.h, the former because it's needed for
the stress test name mangling fix, and the latter for consistency.  As
an artifact of this change, some (but not all) definitions related to
the experimental API are absent from the headers unless the feature is
enabled at configure time.

test/include/test/jemalloc_test.h.in

@@ -102,10 +102,7 @@
  * a separate allocator for their internal data structures.
  */
 #elif defined(JEMALLOC_STRESS_TEST)
-#  define JEMALLOC_NO_DEMANGLE
 #  include "jemalloc/jemalloc@install_suffix@.h"
-#  include "jemalloc/internal/public_unnamespace.h"
-#  undef JEMALLOC_NO_DEMANGLE
 
 #  include "jemalloc/jemalloc_protos_jet.h"
 
@@ -114,8 +111,13 @@
 #  include "jemalloc/internal/public_unnamespace.h"
 #  undef JEMALLOC_JET
 
+#  include "jemalloc/jemalloc_rename.h"
 #  define JEMALLOC_MANGLE
-#  include "jemalloc/jemalloc_mangle@install_suffix@.h"
+#  ifdef JEMALLOC_STRESS_TESTLIB
+#    include "jemalloc/jemalloc_mangle_jet.h"
+#  else
+#    include "jemalloc/jemalloc_mangle.h"
+#  endif
 
 /******************************************************************************/
 /*