Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_from_ubin.c

@@ -0,0 +1,30 @@
+#include "tommath_private.h"
+#ifdef MP_FROM_UBIN_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* reads a uint8_t array, assumes the msb is stored first [big endian] */
+mp_err mp_from_ubin(mp_int *a, const uint8_t *buf, size_t size)
+{
+   mp_err err;
+
+   /* make sure there are at least two digits */
+   if ((err = mp_grow(a, 2)) != MP_OKAY) {
+      return err;
+   }
+
+   /* zero the int */
+   mp_zero(a);
+
+   /* read the bytes in */
+   while (size-- > 0u) {
+      if ((err = mp_mul_2d(a, 8, a)) != MP_OKAY) {
+         return err;
+      }
+      a->dp[0] |= *buf++;
+      a->used += 1;
+   }
+   mp_clamp(a);
+   return MP_OKAY;
+}
+#endif