Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_reduce_2k_setup.c

@@ -0,0 +1,30 @@
+#include "tommath_private.h"
+#ifdef MP_REDUCE_2K_SETUP_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* determines the setup value */
+mp_err mp_reduce_2k_setup(const mp_int *a, mp_digit *d)
+{
+   mp_err err;
+   mp_int tmp;
+
+   if ((err = mp_init(&tmp)) != MP_OKAY) {
+      return err;
+   }
+
+   if ((err = mp_2expt(&tmp, mp_count_bits(a))) != MP_OKAY) {
+      goto LBL_ERR;
+   }
+
+   if ((err = s_mp_sub(&tmp, a, &tmp)) != MP_OKAY) {
+      goto LBL_ERR;
+   }
+
+   *d = tmp.dp[0];
+
+LBL_ERR:
+   mp_clear(&tmp);
+   return err;
+}
+#endif