Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_count_bits.c

@@ -0,0 +1,28 @@
+#include "tommath_private.h"
+#ifdef MP_COUNT_BITS_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* returns the number of bits in an int */
+int mp_count_bits(const mp_int *a)
+{
+   int     r;
+   mp_digit q;
+
+   /* shortcut */
+   if (mp_iszero(a)) {
+      return 0;
+   }
+
+   /* get number of digits and add that */
+   r = (a->used - 1) * MP_DIGIT_BIT;
+
+   /* take the last digit and count the bits in it */
+   q = a->dp[a->used - 1];
+   while (q > 0u) {
+      ++r;
+      q >>= 1u;
+   }
+   return r;
+}
+#endif