Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_abs.c

@@ -0,0 +1,24 @@
+#include "tommath_private.h"
+#ifdef MP_ABS_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* b = |a|
+ *
+ * Simple function copies the input and fixes the sign to positive
+ */
+mp_err mp_abs(const mp_int *a, mp_int *b)
+{
+   mp_err err;
+
+   /* copy a to b */
+   if ((err = mp_copy(a, b)) != MP_OKAY) {
+      return err;
+   }
+
+   /* force the sign of b to positive */
+   b->sign = MP_ZPOS;
+
+   return MP_OKAY;
+}
+#endif