Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_cmp_d.c

@@ -0,0 +1,26 @@
+#include "tommath_private.h"
+#ifdef MP_CMP_D_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* compare a digit */
+mp_ord mp_cmp_d(const mp_int *a, mp_digit b)
+{
+   /* compare based on sign */
+   if (mp_isneg(a)) {
+      return MP_LT;
+   }
+
+   /* compare based on magnitude */
+   if (a->used > 1) {
+      return MP_GT;
+   }
+
+   /* compare the only digit of a to b */
+   if (a->dp[0] != b) {
+      return a->dp[0] > b ? MP_GT : MP_LT;
+   }
+
+   return MP_EQ;
+}
+#endif