Add exhaustive tests for group arithmetic, signing, and ecmult on a small group

If you compile without ./configure --enable-exhaustive-tests=no,
this will create a binary ./exhaustive_tests which will execute
every function possible on a group of small order obtained by
moving to a twist of our curve and locating a generator of small
order.

Currently defaults to order 13, though by changing some #ifdefs
you can get a couple other ones. (Currently 199, which will take
forever to run, and 14, which won't work because it's composite.)

TODO exhaustive tests for the various modules

src/ecmult_impl.h

@@ -13,20 +13,23 @@
 #include "scalar.h"
 #include "ecmult.h"
 
-#include <string.h>
-
-/* optimal for 128-bit and 256-bit exponents. */
-#define WINDOW_A 5
-
 #if defined(EXHAUSTIVE_TEST_ORDER)
+/* We need to lower these values for exhaustive tests because
+ * the tables cannot have infinities in them (this breaks the
+ * affine-isomorphism stuff which tracks z-ratios) */
 #  if EXHAUSTIVE_TEST_ORDER > 128
+#    define WINDOW_A 5
 #    define WINDOW_G 8
 #  elif EXHAUSTIVE_TEST_ORDER > 8
+#    define WINDOW_A 4
 #    define WINDOW_G 4
 #  else
+#    define WINDOW_A 2
 #    define WINDOW_G 2
 #  endif
 #else
+/* optimal for 128-bit and 256-bit exponents. */
+#define WINDOW_A 5
 /** larger numbers may result in slightly better performance, at the cost of
     exponentially larger precomputed tables. */
 #ifdef USE_ENDOMORPHISM