Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_cnt_lsb.c

@@ -0,0 +1,38 @@
+#include "tommath_private.h"
+#ifdef MP_CNT_LSB_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+static const char lnz[16] = {
+   4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
+};
+
+/* Counts the number of lsbs which are zero before the first zero bit */
+int mp_cnt_lsb(const mp_int *a)
+{
+   int x;
+   mp_digit q;
+
+   /* easy out */
+   if (mp_iszero(a)) {
+      return 0;
+   }
+
+   /* scan lower digits until non-zero */
+   for (x = 0; (x < a->used) && (a->dp[x] == 0u); x++) {}
+   q = a->dp[x];
+   x *= MP_DIGIT_BIT;
+
+   /* now scan this digit until a 1 is found */
+   if ((q & 1u) == 0u) {
+      mp_digit p;
+      do {
+         p = q & 15u;
+         x += lnz[p];
+         q >>= 4;
+      } while (p == 0u);
+   }
+   return x;
+}
+
+#endif