Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_lshd.c

@@ -0,0 +1,42 @@
+#include "tommath_private.h"
+#ifdef MP_LSHD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* shift left a certain amount of digits */
+mp_err mp_lshd(mp_int *a, int b)
+{
+   mp_err err;
+   int x;
+
+   /* if its less than zero return */
+   if (b <= 0) {
+      return MP_OKAY;
+   }
+   /* no need to shift 0 around */
+   if (mp_iszero(a)) {
+      return MP_OKAY;
+   }
+
+   /* grow to fit the new digits */
+   if ((err = mp_grow(a, a->used + b)) != MP_OKAY) {
+      return err;
+   }
+
+   /* increment the used by the shift amount then copy upwards */
+   a->used += b;
+
+   /* much like mp_rshd this is implemented using a sliding window
+    * except the window goes the otherway around.  Copying from
+    * the bottom to the top.  see mp_rshd.c for more info.
+    */
+   for (x = a->used; x --> b;) {
+      a->dp[x] = a->dp[x - b];
+   }
+
+   /* zero the lower digits */
+   s_mp_zero_digs(a->dp, b);
+
+   return MP_OKAY;
+}
+#endif