Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_reduce_2k.c

@@ -0,0 +1,49 @@
+#include "tommath_private.h"
+#ifdef MP_REDUCE_2K_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* reduces a modulo n where n is of the form 2**p - d */
+mp_err mp_reduce_2k(mp_int *a, const mp_int *n, mp_digit d)
+{
+   mp_int q;
+   mp_err err;
+   int p;
+
+   if ((err = mp_init(&q)) != MP_OKAY) {
+      return err;
+   }
+
+   p = mp_count_bits(n);
+   for (;;) {
+      /* q = a/2**p, a = a mod 2**p */
+      if ((err = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
+         goto LBL_ERR;
+      }
+
+      if (d != 1u) {
+         /* q = q * d */
+         if ((err = mp_mul_d(&q, d, &q)) != MP_OKAY) {
+            goto LBL_ERR;
+         }
+      }
+
+      /* a = a + q */
+      if ((err = s_mp_add(a, &q, a)) != MP_OKAY) {
+         goto LBL_ERR;
+      }
+
+      if (mp_cmp_mag(a, n) == MP_LT) {
+         break;
+      }
+      if ((err = s_mp_sub(a, n, a)) != MP_OKAY) {
+         goto LBL_ERR;
+      }
+   }
+
+LBL_ERR:
+   mp_clear(&q);
+   return err;
+}
+
+#endif