Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_sqrmod.c

@@ -0,0 +1,15 @@
+#include "tommath_private.h"
+#ifdef MP_SQRMOD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* c = a * a (mod b) */
+mp_err mp_sqrmod(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   mp_err err;
+   if ((err = mp_sqr(a, c)) != MP_OKAY) {
+      return err;
+   }
+   return mp_mod(c, b, c);
+}
+#endif