Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_radix_size.c

@@ -0,0 +1,34 @@
+#include "tommath_private.h"
+#ifdef MP_RADIX_SIZE_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* returns size of ASCII representation */
+mp_err mp_radix_size(const mp_int *a, int radix, size_t *size)
+{
+   mp_err err;
+   mp_int a_;
+   int b;
+
+   /* make sure the radix is in range */
+   if ((radix < 2) || (radix > 64)) {
+      return MP_VAL;
+   }
+
+   if (mp_iszero(a)) {
+      *size = 2;
+      return MP_OKAY;
+   }
+
+   a_ = *a;
+   a_.sign = MP_ZPOS;
+   if ((err = mp_log_n(&a_, radix, &b)) != MP_OKAY) {
+      return err;
+   }
+
+   /* mp_ilogb truncates to zero, hence we need one extra put on top and one for `\0`. */
+   *size = (size_t)b + 2U + (mp_isneg(a) ? 1U : 0U);
+
+   return MP_OKAY;
+}
+#endif