Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_to_sbin.c

@@ -0,0 +1,22 @@
+#include "tommath_private.h"
+#ifdef MP_TO_SBIN_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* store in signed [big endian] format */
+mp_err mp_to_sbin(const mp_int *a, uint8_t *buf, size_t maxlen, size_t *written)
+{
+   mp_err err;
+   if (maxlen == 0u) {
+      return MP_BUF;
+   }
+   if ((err = mp_to_ubin(a, buf + 1, maxlen - 1u, written)) != MP_OKAY) {
+      return err;
+   }
+   if (written != NULL) {
+      (*written)++;
+   }
+   buf[0] = mp_isneg(a) ? (uint8_t)1 : (uint8_t)0;
+   return MP_OKAY;
+}
+#endif