Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_rand.c

@@ -0,0 +1,39 @@
+#include "tommath_private.h"
+#ifdef MP_RAND_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+mp_err mp_rand(mp_int *a, int digits)
+{
+   int i;
+   mp_err err;
+
+   mp_zero(a);
+
+   if (digits <= 0) {
+      return MP_OKAY;
+   }
+
+   if ((err = mp_grow(a, digits)) != MP_OKAY) {
+      return err;
+   }
+
+   if ((err = s_mp_rand_source(a->dp, (size_t)digits * sizeof(mp_digit))) != MP_OKAY) {
+      return err;
+   }
+
+   /* TODO: We ensure that the highest digit is nonzero. Should this be removed? */
+   while ((a->dp[digits - 1] & MP_MASK) == 0u) {
+      if ((err = s_mp_rand_source(a->dp + digits - 1, sizeof(mp_digit))) != MP_OKAY) {
+         return err;
+      }
+   }
+
+   a->used = digits;
+   for (i = 0; i < digits; ++i) {
+      a->dp[i] &= MP_MASK;
+   }
+
+   return MP_OKAY;
+}
+#endif