Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_dr_is_modulus.c

@@ -0,0 +1,27 @@
+#include "tommath_private.h"
+#ifdef MP_DR_IS_MODULUS_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* determines if a number is a valid DR modulus */
+bool mp_dr_is_modulus(const mp_int *a)
+{
+   int ix;
+
+   /* must be at least two digits */
+   if (a->used < 2) {
+      return false;
+   }
+
+   /* must be of the form b**k - a [a <= b] so all
+    * but the first digit must be equal to -1 (mod b).
+    */
+   for (ix = 1; ix < a->used; ix++) {
+      if (a->dp[ix] != MP_MASK) {
+         return false;
+      }
+   }
+   return true;
+}
+
+#endif