Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_mod_2d.c

@@ -0,0 +1,40 @@
+#include "tommath_private.h"
+#ifdef MP_MOD_2D_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* calc a value mod 2**b */
+mp_err mp_mod_2d(const mp_int *a, int b, mp_int *c)
+{
+   int x;
+   mp_err err;
+
+   if (b < 0) {
+      return MP_VAL;
+   }
+
+   if (b == 0) {
+      mp_zero(c);
+      return MP_OKAY;
+   }
+
+   /* if the modulus is larger than the value than return */
+   if (b >= (a->used * MP_DIGIT_BIT)) {
+      return mp_copy(a, c);
+   }
+
+   if ((err = mp_copy(a, c)) != MP_OKAY) {
+      return err;
+   }
+
+   /* zero digits above the last digit of the modulus */
+   x = (b / MP_DIGIT_BIT) + (((b % MP_DIGIT_BIT) == 0) ? 0 : 1);
+   s_mp_zero_digs(c->dp + x, c->used - x);
+
+   /* clear the digit that is not completely outside/inside the modulus */
+   c->dp[b / MP_DIGIT_BIT] &=
+      ((mp_digit)1 << (mp_digit)(b % MP_DIGIT_BIT)) - (mp_digit)1;
+   mp_clamp(c);
+   return MP_OKAY;
+}
+#endif