Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_neg.c

@@ -0,0 +1,18 @@
+#include "tommath_private.h"
+#ifdef MP_NEG_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* b = -a */
+mp_err mp_neg(const mp_int *a, mp_int *b)
+{
+   mp_err err;
+   if ((err = mp_copy(a, b)) != MP_OKAY) {
+      return err;
+   }
+
+   b->sign = ((!mp_iszero(b) && !mp_isneg(b)) ? MP_NEG : MP_ZPOS);
+
+   return MP_OKAY;
+}
+#endif