Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_div_2.c

@@ -0,0 +1,40 @@
+#include "tommath_private.h"
+#ifdef MP_DIV_2_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* b = a/2 */
+mp_err mp_div_2(const mp_int *a, mp_int *b)
+{
+   mp_err err;
+   int x, oldused;
+   mp_digit r;
+
+   if ((err = mp_grow(b, a->used)) != MP_OKAY) {
+      return err;
+   }
+
+   oldused = b->used;
+   b->used = a->used;
+
+   /* carry */
+   r = 0;
+   for (x = b->used; x --> 0;) {
+      /* get the carry for the next iteration */
+      mp_digit rr = a->dp[x] & 1u;
+
+      /* shift the current digit, add in carry and store */
+      b->dp[x] = (a->dp[x] >> 1) | (r << (MP_DIGIT_BIT - 1));
+
+      /* forward carry to next iteration */
+      r = rr;
+   }
+
+   /* zero excess digits */
+   s_mp_zero_digs(b->dp + b->used, oldused - b->used);
+
+   b->sign = a->sign;
+   mp_clamp(b);
+   return MP_OKAY;
+}
+#endif