Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_from_sbin.c

@@ -0,0 +1,21 @@
+#include "tommath_private.h"
+#ifdef MP_FROM_SBIN_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* read signed bin, big endian, first byte is 0==positive or 1==negative */
+mp_err mp_from_sbin(mp_int *a, const uint8_t *buf, size_t size)
+{
+   mp_err err;
+
+   /* read magnitude */
+   if ((err = mp_from_ubin(a, buf + 1, size - 1u)) != MP_OKAY) {
+      return err;
+   }
+
+   /* first byte is 0 for positive, non-zero for negative */
+   a->sign = (buf[0] != (uint8_t)0) ? MP_NEG : MP_ZPOS;
+
+   return MP_OKAY;
+}
+#endif