Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/s_mp_get_bit.c

@@ -0,0 +1,21 @@
+#include "tommath_private.h"
+#ifdef S_MP_GET_BIT_C
+
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* Get bit at position b and return true if the bit is 1, false if it is 0 */
+bool s_mp_get_bit(const mp_int *a, int b)
+{
+   mp_digit bit;
+   int limb = b / MP_DIGIT_BIT;
+
+   if (limb < 0 || limb >= a->used) {
+      return false;
+   }
+
+   bit = (mp_digit)1 << (b % MP_DIGIT_BIT);
+   return ((a->dp[limb] & bit) != 0u);
+}
+
+#endif