Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_complement.c

@@ -0,0 +1,13 @@
+#include "tommath_private.h"
+#ifdef MP_COMPLEMENT_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* b = ~a */
+mp_err mp_complement(const mp_int *a, mp_int *b)
+{
+   mp_int a_ = *a;
+   a_.sign = ((a_.sign == MP_ZPOS) && !mp_iszero(a)) ? MP_NEG : MP_ZPOS;
+   return mp_sub_d(&a_, 1uL, b);
+}
+#endif