Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_invmod.c

@@ -0,0 +1,29 @@
+#include "tommath_private.h"
+#ifdef MP_INVMOD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* hac 14.61, pp608 */
+mp_err mp_invmod(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   /* for all n in N and n > 0, n = 0 mod 1 */
+   if (!mp_isneg(a) && mp_cmp_d(b, 1uL) == MP_EQ) {
+      mp_zero(c);
+      return MP_OKAY;
+   }
+
+   /* b cannot be negative and has to be >1 */
+   if (mp_isneg(b) || (mp_cmp_d(b, 1uL) != MP_GT)) {
+      return MP_VAL;
+   }
+
+   /* if the modulus is odd we can use a faster routine instead */
+   if (MP_HAS(S_MP_INVMOD_ODD) && mp_isodd(b)) {
+      return s_mp_invmod_odd(a, b, c);
+   }
+
+   return MP_HAS(S_MP_INVMOD)
+          ? s_mp_invmod(a, b, c)
+          : MP_VAL;
+}
+#endif