Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/s_mp_log_2expt.c

@@ -0,0 +1,12 @@
+#include "tommath_private.h"
+#ifdef S_MP_LOG_2EXPT_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+int s_mp_log_2expt(const mp_int *a, mp_digit base)
+{
+   int y;
+   for (y = 0; (base & 1) == 0; y++, base >>= 1) {}
+   return (mp_count_bits(a) - 1) / y;
+}
+#endif