Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_signed_rsh.c

@@ -0,0 +1,21 @@
+#include "tommath_private.h"
+#ifdef MP_SIGNED_RSH_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* shift right by a certain bit count with sign extension */
+mp_err mp_signed_rsh(const mp_int *a, int b, mp_int *c)
+{
+   mp_err err;
+   if (!mp_isneg(a)) {
+      return mp_div_2d(a, b, c, NULL);
+   }
+
+   if ((err = mp_add_d(a, 1uL, c)) != MP_OKAY) {
+      return err;
+   }
+
+   err = mp_div_2d(c, b, c, NULL);
+   return (err == MP_OKAY) ? mp_sub_d(c, 1uL, c) : err;
+}
+#endif