Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_reduce_is_2k.c

@@ -0,0 +1,34 @@
+#include "tommath_private.h"
+#ifdef MP_REDUCE_IS_2K_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* determines if mp_reduce_2k can be used */
+bool mp_reduce_is_2k(const mp_int *a)
+{
+   if (mp_iszero(a)) {
+      return false;
+   } else if (a->used == 1) {
+      return true;
+   } else if (a->used > 1) {
+      int ix, iy = mp_count_bits(a), iw = 1;
+      mp_digit iz = 1;
+
+      /* Test every bit from the second digit up, must be 1 */
+      for (ix = MP_DIGIT_BIT; ix < iy; ix++) {
+         if ((a->dp[iw] & iz) == 0u) {
+            return false;
+         }
+         iz <<= 1;
+         if (iz > MP_DIGIT_MAX) {
+            ++iw;
+            iz = 1;
+         }
+      }
+      return true;
+   } else {
+      return true;
+   }
+}
+
+#endif