Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_rshd.c

@@ -0,0 +1,43 @@
+#include "tommath_private.h"
+#ifdef MP_RSHD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* shift right a certain amount of digits */
+void mp_rshd(mp_int *a, int b)
+{
+   int x;
+
+   /* if b <= 0 then ignore it */
+   if (b <= 0) {
+      return;
+   }
+
+   /* if b > used then simply zero it and return */
+   if (a->used <= b) {
+      mp_zero(a);
+      return;
+   }
+
+   /* shift the digits down.
+    * this is implemented as a sliding window where
+    * the window is b-digits long and digits from
+    * the top of the window are copied to the bottom
+    *
+    * e.g.
+
+    b-2 | b-1 | b0 | b1 | b2 | ... | bb |   ---->
+                /\                   |      ---->
+                 \-------------------/      ---->
+    */
+   for (x = 0; x < (a->used - b); x++) {
+      a->dp[x] = a->dp[x + b];
+   }
+
+   /* zero the top digits */
+   s_mp_zero_digs(a->dp + a->used - b, b);
+
+   /* remove excess digits */
+   a->used -= b;
+}
+#endif